My favorite free antivirus program is Avira, from
http://www.free-av.com There is also AVG, at
http://free.avg.com, but with their last program update, it got rather bloated. Avira is much less of a drain on system resources.
Most anti-virus programs, free or not, have a version you can download and install. Usually even the non-free ones have a free trial you can install - you basically pay for a subscription to the updates. (And yes, you really do need updates. Otherwise the software won't detect newly developed viruses. I can't tell you how many computers I've disinfected where the owner thought they were protected because they had AV software installed, but hadn't updated it in a year.)
From some of the reviews I've read, Avast and Avira are pretty close as far as not slowing down a system, but Avast has slightly more false positives. (saying a file is a virus when it isn't.)
If you decide to pay for AV software, stay away from Symantec/Norton. The Symantec Corporate edition was pretty good last time I used it, but their home-use software will drag your PC to a halt. I use Trend-Micro at work, and have been satisfied, but I've got several friends that swear by NOD32, and I'm thinking of trying it out at work next time our Trend-Micro subscription is up.