Microsoft does a good job at providing tips and tricks for visitors to their website. The Windows XP portion of their site has an Expert Zone Community with all sorts of great help. They also have other tips that are posted from time to time. Two notable ones are:
5 cool Windows XP tips
http://www.microsoft.com/windowsxp/homeusers/articles/5tips.mspx
4 Ways to Speed Up Your Computer’s Performance
http://www.microsoft.com/AtWork/getstarted/speed.mspx