I agree with jmortensen. I like spybot first, then ad-aware second. I recommend running both programs and their updates at least once a week. When I was a tech support rep, we would use both programs to remove spyware on our clients' computers. You can run both programs on your computer as long as you have windows 2000 and above (otherwise, install one or the other).
A friend of mine installed both programs on a windows 95 or 98 (i forget) computer. All spyware was removed, however the following day the internet connection was not working. After running a few tests, as well as that the ISP was fine, we uninstalled both programs. Afterwards, the connection was back and running.