Personally, I don't use one, I just do an online scan from time to time to find nothing. ;-) But, on computers where I'm not the only user, I used to install AVG Free 7.5. The problem is that it's discontinued and AVG Free 8.0 is a real resource hog.
I tried avast! with the basic minimumm scanners, following recommendations from topics about anti-virus comparisons, since it's apparently a resource hog otherwise. I also heard that it doesn't remove viruses extremely well, but that it was caused by older versions not having the on-access scanner running by default.
The next I tried is AntiVir. It's quite light (doesn't take 45 seconds to boot unlike AVG 8) and apparently is one of the AVs that finds the most viruses, but there are two big disadvantages with the free version : The update servers are so slow it takes minutes to download a 100kB file from them, and there are popups showing up while it's updating.
I read in a few places on the subject, and avast! with a minimum of options enabled (just the standard AV scanner) and AntiVir are good choices.
There is one last and very good choice, but only if you don't need to use the "Switch User" feature : Comodo Anti-Virus. It's free, updates are automatic and fast, it doesn't slow down the computer and does e-mail scanning. I'd recommend disabling the Application Control features, as it asks you if you want to allow or disallow every EXE, SYS, etc. from executing. It's quite annoying as every times you update an application, it asks you again, and again,... I read an anti-virus review once, and Comodo AV was the only one that "does everything well".