From the MSN Tech site.

Basically it comes down to clearing out the accumulated rubbish – unneccessary programmes and services that run automatically at start-up.

I would add deleting any fonts you don’t use, and defragmenting your hard disk every month or so.

Depending on your current configuration, adding more memory can also make a big difference – up to 1GB for XP Home, 2GB for XP Pro, and 4GB for Vista.