I used to follow the reinstall windows every so often philosophy, especially before XP came out.  But after reinstalling XP 5 times in one week and 100's of GB's of software (due to an impending hardware failure that I finally diagnosed after the 5th load), along with reregistration, entering software keys, etc.,  I swore I would never do it again.

I picked up JV 16 Power Tools (not free) which is a suite of tools.  I use the Registry Cleaner the most. It is amazing how many useless and obsolete keys, file pointers and other junk build up in only a week, especially if you copy, move and delete a lot of files, install and uninstall software.

Adware, spyware, etc, in my opinion, fall into a different category, with their own tool sets for prevention and removal.  I used to use Norton on my ME machine years ago, and it saved me from a trojan horse, once, but I still had to go into the registry to remove the final traces that Norton couldn't remove.  I now use a different strategy, with tool sets for both general maintenance and special maintenance.

I routinely clean out my working folders, run registry cleaner and then defrag my hard drive every week or two.  That with having disabled all windows services I don't need has kept my 'puter running lean and mean for over a year since I built it.  I consider this general maintenance.