I just use good old Spybot S&D, and Ccleaner on a rotating basis
once a month.

I also run an AVG scan maybe twice a week.

Never have any probs.

if ive installed some dubious software i might run a few scans on it, but apart from that, Im always clean....I fix computers in my spare time, and 99% of the time computers run slow because the owners just dont care about maintenance.

A clasic one I had the other day, Spybot picked up about 8 probems, one of which had over 600 instances of some malware...

needless to say it ran a bit better with out the malware lol.