Rob Pegoraro of The Washington Post has an excellent column about two freeware antivirus programs: Avast! and AVG. As a former user of AVG and current user of Avast!, I thought I'd add a little anecdotal information to his testing of the programs.
I used AVG for at least four years. It worked admirably. Then they released a new version of the program, and stopped supporting the virus database for the older version. The new version simply slowed my computer to a crawl (I'm using a 200mhz Pentium with 128meg ram), and sometimes locked it up, or provided the Blue Screen of Death.
I switched to Avast! at that time, and haven't had any problems since. But I do agree with Rob that the interface for AVG is much superior to that of Avast! My parents run AVG on their computer (600mhz Pentium, I forget how much ram), and it's working well for them.
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