I figured I do a quick test on a few of the most popular free anti-virus programs out and see which really uses the most resources. Test Method: I used a fresh install of Windows 7 x64 Pro. After installing Win7, drivers, and all Windows Updates I made an image of the machine. Then I installed each anti-virus, re-applying the image between anti-viruses to make sure each one gets a equally clean slate. After the AV was installed, I let the computer sit for 30 minutes to make sure it goes through its initial update process. The AVs were idle when I took the readings, these are not scan resources, but your scan should be set to run when the computer isn't in use anyway. The resources I'm worried about is the RAM usage of each program. To figure this out I used ProcessXP which just makes it easy to pick up what processes belong to the AV software. Results: Column 1 Column 2 0 AVG 81MB 1 Avast 34MB 2 Avira 110MB 3 Comodo 145MB 4 MSE 158MB Short and sweat. But really, any system with reasonable amounts of RAM(2GB or more) probably won't even notice the difference.