I go with AVG - worked well for three years. They also now do antispyware. The only thing about AVG which confuses people is that when there is a new version there is a message which implies (to some people) that you now have to pay. This is not true - you just have to find the AVG free website and download the latest versions.
I have tried spamfighter and zonealarm in the past, both caused great problems. I have also had paid for McAfee and Norton. In my opinion not worth it as they cause even more conflicts with various quite normal software. But I am behind a firewall on my Draytek vigor wireless router. The free ones are so good that I see no point in paying for bloated suites which mess things up.
I have use adaware Se (free) as well as use cCleaner every other day - amazing how many malignant cookies it finds.
Ask Jack at Guardian Online is good:-
http://blogs.guardian.co.uk/askjack/cat-271/The problem with some of the "free" sites which check/analyse your computer is that they give exaggerated reports so that you are scared into buying their solutions. There are articles about this somewhere on the link above.
Oh, and the AVG free (including their new rootkit)/ccleaner/using adaware se as ell periodically was is recommended by my neighbours who build and trouble-shoot computers.