After testing a bunch of different anti virus packages I have found Avast to be the least intrusive.
My test was simple. I had an older laptop running Windows XP. I would start the laptop, open Firefox, browse to USAToday and let the page load. Then close Firefox. Open Word, enter text, then close Word, then open IE. Here were my results.
No Protection took 5:18
Avast took 10:17
Microsoft Security Essentials took 14:10
AVG took 14:26
Symantec took 17:12
