AVG has slipped in the ratings. A lot. I used it for several years and have recently switched to ESET NOD32. After getting infected. Google 'antivirus review' and check out some of the independent evaluation sites. NOD32 and Kaspersky pretty generally beat everybody else.
You can get Spyware Doctor for free by going to the Google site, clicking on 'more' and 'even more' until you get to the end, and then click on 'Pack.' Select Spyware Doctor (or deselect everything else you don't want, I don't remember which).
Ad-Aware and Spybot S&D are both good (different ways of looking for malware produces different results) and both have free versions. Only catch is you have to remember to run them from time to time. Weekly is okay.
--John
(Oh, and I advise that you stay away from suites like McAfee and Norton. They're resource hogs -- that is, they'll bog down your computer speed bad -- and while they may be okay for one security task, they don't do everything well. Besides, their pricing is not user-friendly.)