My GF just got hooked to the internet and I put Avast! and Ad-ware on it for her. Both are free and quite good, but Ad-ware isn't realtime checking (you have to run it manually every so often). Of the free ones, Avast! is great.
On my new vista machine (and carried over from my old XP machine) I use BitDefender. No problems and seems to work quite well. They even provided a new Vista version for free.
Before BitDefender I had PC-cillin. Loved it- at first. But after several months my machine kept getting slower and slower. Did all the usual stuff (defrag, registry clean, etc) but no luck. Testing to see what could be causing the slowdown I turned off PC-cillin and it flew like it was new. Nuff said. That's why I got BitDefender and so far no slowdowns.
FYI, if you have an XP machine, I highly recommend getting CCleaner. Lets you keep the cookies you want, and deletes the rest, plus cleaning up all the extra junk, and it's registry cleaner is quite good (but not great). WARNING: The latest version says it's good for Vista, but it's not (don't ask!). They update CCleaner all the time, so I'll wait a few more versions before putting it back on my vista machine (I really miss that little program!)