Hiya
As a general rule of thumb, I defrag once a month.
I do this as the very last thing, mind.
I have Interet Explorer set up as follows. under Internet Tools | Advanced Settings, under Security, I have Empty Tempory Internet Files when browser is closed.
I empty my temp folder out once a week, run AddAware
www.lavasoftusa.org as well as RefUpdate to keep the program uptodate. I do this (addAware) daily.
I run my AV scanner daily, just to be sure. And thats about it.
Oh, and keep those Startup programs down to a minimum.
Have a read of this as well:
http://www.piersontech.com/07-CleanHardDrive.htm
Regards
eddie