caution

Sounds like you are a beginner. The safe way to do it, is to get another hard drive, save you old one in case things go wrong you can always swap it back. if you are reinstalling XP, not much can go wrong. If you are upgrading from 98 or 2000, that is another story. If you are unlucky, you may not find some of the drivers for your new OS, and you will be stuck. These days, it is generally cheaper and less trouble to buy a new computer for $300 (from dell) than trying to upgrade, if your computer is more than 4 years old.