A few months ago I declared: "It's time to toss out your (desktop) antivirus software!" As far as I was concerned, running desktop antivirus software was a liability in and of itself because "Running antivirus on a personal computer is like having the bomb squad inspect a suspicious package inside the house right next to you." The effectiveness of antivirus software is also questionable since it won't work at all for zero-day exploits that haven't been updated yet. Well now there seems to be another good reason to toss out that antivirus software.
Everyone has always suspected antivirus software of slowing computers down (at least through anecdotal evidence), but no one has ever been able to really quantify it precisely. A young English gentleman in the UK who goes by "Oli" has posted this wonderful analysis on "What really slows Windows down" and posted some detailed measurements on the effects of typical desktop software and security suites.
The desktop Antivirus suites all appear to make your PC run slower than a 5 year old computer when it comes to slowing hard drive I/O down which is the biggest factor in PC wait times. Norton Internet Security 2006 was the worst resource hog, McAfee VirusScan Enterprise 8 was the second worst, but Norton Internet Security 2007 seemed to have improved to the third worst resource hog. Trend Micro PC-cillin AV 2006 was the fourth worst resource hog and Microsoft's Live OneCare had significantly lower overhead. Surprisingly, AVG 7.1 free antivirus software came in with extremely low overhead compared to any of the other Antivirus suites so if you must run something, AVG might be the way to go and you certainly can't argue with the price.
As anyone who knows me would know, I personally never use Antivirus or Anti-spyware software and neither has most of my expert friends or colleagues and we never get viruses even while running as full administrator. When my family members use the computer, I set them to standard users and the worst I'll ever need to do is nuke their account and recreate it if something bad happens. I'm also careful to only give them read only access to family photos and files so that they can't ever accidentally delete them or click on some Malware that would delete them. Now how do I know I don't have any viruses? I do manually conduct occasional scans of the hard drive for viruses and spyware and I never find any.