Well I recommend you defrag (and use the disk clean up utility) once a month. Many say that it doesn't work as well, and use utilities like Perfect Disk (you need to pay for it) or Diskeeper 10 (disk keeper is really awesome...but you have to pay for it as well). Edit: You can also get Power Defragmenter, which is also apparently good.

There are two ways of helping you remember when to defrag, either you can remind yourself (try e-mailing youself using services like Future me) or use task scheduler (this will do it for you automatically).

Actually I'll write up an article or something of how to do is using task scheduler, but here's the basics of how to do it (in Xp):

1. Open task scheduler (Start > All programs > Accesories > system tools > Scheduled tasks)
2. Click on add scheculed task
3. Just follow the instructions of the Scheduled Task Wizard

and then windows will automatically run the program at the time you specified...

Hope this helps