You probably don’t need it most days (but, you’ll never know which days). In fact, you might or might not encounter a “drive by” for days or weeks. But, AV-Comparatives (AVC) did an interesting test on AVs (Antivirals). They tested when the AV actually loads. That’s not to say they’re actually protecting you during boot, but AVC didn’t actually test that.

Another caveat: They tested on new machines. That’s not exactly real world testing.

“To test a product's effect on day-to-day computer use, the researchers timed a number of common activities with no antivirus at all, averaging several runs, and then re-tested in the same way with the antivirus running. The tests included: copying files between drives; zipping and unzipping files; installing and uninstalling applications; transcoding music files; launching applications; and downloading files.”

- http://securitywatch.pcmag.com/security-software/311241-speed-up-pc-performance-by-replacing-microsoft-s-built-in-antivirus

They found a big difference in system impact between some AV’s. One didn’t load until eleven minutes after boot initiation.

A philosophical difference: Some AVs would allow the virus to load and then whack it. I can’t agree with that approach. The malware should be whacked ASAP to prevent your having to repair services, etc.

So how much does each AV affect boot time? The sooner it boots, its effect will be felt. That might decrease on subsequent scans, both a good thing and under some circumstances a bad one since files flagged as safe might become infected. Other factors affecting boot time are age of computer, amount of filling of the hard disk, speed of processor, degree of fragmentation and amount of RAM will affect the boot speed as well. So, the AV probably doesn’t determine a heck of a lot when it comes to boot speed. It does affect a great deal with respect to boot safety. So, maintaining a lean, well tended to machine determines a great deal.

Here’s a screen shot of their results:

On this graph, the closer to the left the better.From BitDefender to the left, the AV’s earned the Advanced+ score.

AVC found that about one-third of the products tested affected boot time less than MSE (Microsoft Security Essentials). In fact, You might just boot faster with one of the Advanced+ AVs and without MSE which hasn’t been doing well at all in AVC’s testing.

Source:

http://securitywatch.pcmag.com/security-software/311241-speed-up-pc-performance-by-replacing-microsoft-s-built-in-antivirus