I've had many calls from both my mom and dad asking be why the virus scanner has a message up. If your running Windows without a virus scanner and you browse the web on this PC your taking a big risk on having something unwanted installed. Just do a google search for website hacked installing malware for a reason to run a virus scanner.

A good Virusscaner shouldn't make your computer slow nor should it make it crash. I run NOD32 on all of my Windows computers and the only issue I've ever seen was on a friends computer. He was unable to close iTunes and the fix was very simple. Just add iTunes.exe to the do not active scan list.