I used to really like AVG Free, but the last time I put it on my computer it subsequently crashed moments after running the scan. Related? I can't be sure . . it's either AVG on a response virus that caused it. Who knows!
I use Norton and Spyware Doctor, and since using both I have yet to have a problem. I think its important to have at least 1 antivirus/antispyware that you pay, I think they work better. I have also heard of many people getting spyware from antispyware download pages... but who knows. I don't like the fact that some antispyware software is starting to block affiliate cookies though, scary stuff!
I am using Adware which seems pretty good but my computer is still running pretty slow. I guess I'll use the other ones as well. Does Mozilla get less viruses then internet explorer?
I've found Spy Sweeper to be the best of the competition. It offers a variety of scans with up to 15 different shields and an incredibly ease of use. The only disadvantage is that if you have an older computer, it may run kind of slow. If you have the power though, there is no reason to not go with it.