There's a lot of stuff that you can do. Here are a few things, I always do:
- Run avg antivirus
- Use a regular user for day to day use and use the admin account for management of the os
- Adjust the system settings to run in classic mode and turn off all the non-essential eye candy
Here's some more stuff to help you get started:
http://www.microsoft.com/windowsxp/using/setup/tips/default.mspxhttp://www.tweakxp.com/performance_tweaks.aspxhttp://www.tweakguides.com/TGTC.htmlTweak guides has a very detailed guide.