Another happy Avast user, here.
The "Big Three" freebies (or "have free version") AV's are
AVG (they also have a free version-non resident- of their antispyware, which has a good rep),
Avast Home ,and
Avira Classic They are all good. All generally score pretty well at
http://www.av-comparatives.org/, which is probably considered
the definitive AV test site. They often seem to achieve better results than a lot of the paid offerings, but be advised, it is only the "pro" versions of the AV's that get tested there.
One of the things about Avast is that the detection and cleaning ability of the Home version (free) is the same as for the pro. Not necessarily the case with the others. I also find the program very stable, very configurable, and skinnable, with excellent forum support for a freebie.
Avira consistently seems to achieve the best detection rates, users generally like it a lot, one little drawback is a nag screen that pops up every time you update. AVG is probably the most popular. I've tried all three. The computer is protected by only freeware applications,(AV, AS, Hosts file, 2 way Firewall, Noscript) and I haven't had anything more fearsome than a tracking cookie for years.
In the end, you'll likely be well protected with any of them, especially if you include a two way firewall, and your choice might as well be down to which you like the look of, or functionality, or ease of use etc, the most.