Dedicated to Giz who is being dragged bodily into Windows 7 and likes not one minute of it.

Microsoft's support for Windows XP may be fading, but a loyal horde of XP users plans to stick with this venerable OS for as long as possible.

If that's your long-term goal, there are a number of steps you can take now to ensure a finely tuned XP system for months — possibly years — to come.
Windows XP is almost a decade old, which in both computing and dog years makes it very long in the tooth.
Microsoft has officially dropped support and security updates for all XP versions through Service Pack 2. The only version of 32-bit XP that still qualifies for Microsoft's security patches and major bug-fixes is the Service Pack 3 edition. (The relatively rare 64-bit flavor of XP is a special case. See Microsoft's explanation.)
XP has had a long and excellent run, but SP3 is the end of the line.
That said, XP is not dead, and it's still the best OS for older hardware designed with XP in mind. (I have XP on several of my older systems.)
If you're still using an XP box by choice (or necessity), there's lots you can do to keep things humming along until you eventually move to new hardware — which will almost assuredly come with the excellent Windows 7 already installed.
Here are some key steps you can take to get — and keep — your XP system running great! And if you move to Windows 7 (or are also running Vista machines), many of these techniques can also help you.

Start with a thorough XP system checkup:

Check the hardware. Hardware? Yes! No operating system can be better than the hardware on which it's installed, and older systems are prone to age-related problems. One often-overlooked problem is dust buildup, which can cause chips and drives to overheat and malfunction. These hardware errors can masquerade as software problems, causing you to waste time troubleshooting the wrong thing.
It's easy to clean your PC. Consult the how-to article, "Getting the grunge out of your PC." (It's a few years old, but still completely apt.) While you have your PC's case open, make sure that all plug-in cards and socketed chips are fully seated and all cables firmly connected.
 

Check your hard drive's "physical" health. Most new and XP-era drives are equipped with Self-Monitoring, Analysis and Reporting Technology, also known as SMART reporting. SMART data is stored within the hard drive itself and can often alert you to impending problems before they get serious.
It's easy to check the SMART data. Two good tools are PassMark's DiskCheckup (info/download page) and Active@ DiskMonitorFree (download page). Both programs are free for personal use and also come in commercial versions for organizations.
 

Check your hard drive's "logical" health. Run chkdsk.exe to check the integrity of your hard drive's files and to repair any errors.
Click Start and Run, then type chkdsk c: /f into the Run dialog box. Hit OK.
Chkdsk may tell you that it can't check the drive because the drive is in use. It will then offer to check the drive at reboot. Type Y (yes) and hit the Enter key.
Repeat for all drives/partitions on your system.
 

Correct driver errors now, while you can. Just as Microsoft is providing less support for XP, third-party vendors are withdrawing support for older hardware. Someday soon, you may discover that the drivers you need are no longer available. Fix problems now! So save a copy of all on an external storage device as well or on a CD or DVD.
Boot XP and right-click My Computer. Select Properties, Hardware, then Device Manager. (Or, click Control Panel/System/Hardware/Device Manager.) Click View and select Show hidden devices to make sure you're seeing everything.
Correct any problem indicated by a yellow exclamation mark or a red X; in most cases, you should get correct or updated drivers from the hardware vendor's site.
It might also be wise to save copies of any special drivers your systems needs; burn 'em to a CD or DVD, and tuck the disc away in a safe place.
 

Review and update your PC's security system!

Patch and update XP and apps. Starting with Windows Update, make sure your operating system is fully up-to-date with all necessary patches, fixes, and updates. Do the same for all your non-Microsoft software, visiting the vendor sites to download any new updates and patches for your applications and utilities. A tool such as Secunia's outstanding, free-for-home-use Personal Software Inspector (PSI) (download page) can make this step a breeze.

Verify system security. Regardless of the antivirus and anti-malware tool(s) you're using, visit a competing vendor's site and run their free live or online scan to verify that nothing slipped past your usual defenses.
Next, check that your firewall is providing the protection it should. There are many good, free, online firewall-test sites, such as Hackerwatch, Gibson Research ShieldsUP, and AuditMyPC.

Give your computer a thorough file cleaning

Take out the trash — all of it. Needless file clutter makes a system harder to use and slower to operate. For example, AV scans and Windows' indexing both take longer when they have many junk files to process.
Start by deleting old $NtUninstall{xxx}$ files from XP's C:\Windows folder; these files can occupy a shocking amount of space! You need these files only when a Windows Update fails and you (or the OS) have to roll back your system. If your system is working fine, $NtUninstall files serve no purpose.
Next, wade through your hard drive, folder by folder, making sure files are where they're supposed to be and that you're not storing needless duplicates or other useless files.
Next, uninstall obsolete or unused software.
Finally, use a tool such as Piriform's free CCleaner (site) to rid your drive of useless junk files and broken or obsolete Registry data.

Rein in XP's three worst space-hogs. System Restore, the Recycle Bin, and browser caches are like black holes for data, and your system can run better if you limit their voracious appetites.

System Restore is at best a limited recovery tool, so I don't feel it's worthwhile to devote vast amounts of disk space to it. The Kellys-Korner article, "System Restore for Windows XP," tells you how to manage it.
Windows' default Recycle Bin can consume hundreds of gigabytes on a large drive. Pare this down to a reasonable size by right-clicking the Recycle Bin and selecting Properties. Reduce the size of the Recycle Bin to a smaller percentage of the total disk space. (Click the disk tab — e.g., Local Disk (C:) — to determine its reserved Recycle Bin space in gigabytes.) I set it to around 500 MB (0.5GB) on large disks and 250MB (0.25GB) on smaller ones.
To reduce Internet Explorer's cache size, click Tools and Internet Options. Then, under the Browsing History section, click Settings and adjust the cache size downward to, say, 50MB.
For Firefox, click Tools/Options and then click Advanced. Under the Network tab, look for the settings box in the Offline Storage section.
Chrome's cache-size adjustment uses the command line, as described on a Chrome Help forum page.

Defrag. Once your disk is rid of all unnecessary files and is organized the way you want, run your defragmentation tool to reorder your files for optimal performance. If your disk was badly fragmented, it may take several iterations of defragging to achieve maximum benefit. Defrag your RAM as well.

Use disk imaging to preserve your new setup!
Once you've worked through all the above, your XP system should be lean, clean, defragged, and fully up-to-date. Wouldn't it be great if you could somehow preserve your PC's current software state so that, should you ever need to in the future, you can bring it back to this nearly perfect condition in just minutes?

You can! Use a disk imaging tool  (I prefer Acronis) to create a perfect, complete, working copy of your current setup. You'll never again have to rebuild your system and reinstall all your software from scratch!

XP requires third-party disk-imaging software (Win7 has it built in) such as Acronis' U.S. $30 True Image (info page), Norton' $70 Ghost (site), or — another good one for non-Win7 systems — Terabyte Unlimited's geeky-but-powerful $35 BootItNG (info page).
All three programs make disk images and bootable recovery discs that can be used to restore a complete, everything-installed-and-working setup — even to a raw, unformatted drive.
There's plenty of free disk imaging software available, too. For example, see Freebyte's page titled "Free disk image software;" TheFreeCountry's list of "Free hard disk and partition imaging and backup software;" or OptimizingPC's how-to, "Create free bootable Windows XP image disk."
Run through the above steps once or twice a year to keep your system in tip-top shape, and make a fresh disk image from time to time — especially if you make any significant changes to your hardware or software. Store your disk images in a safe place (off the hard drive), such as on CDs or DVDs stored away from your PC.

With this kind of routine maintenance, your XP system will most likely run well for as long as you need it. And, should the worst (major crash, hard drive failure, etc.) happen, you can use your disk images to rapidly restore your system to the near-perfect state you just created.

You're now set for the long haul!