It depends on whether or not you have a full-fledged Windows disc or just a "restore" disc from your PC manufacturer. If you have a regular Windows CD, you boot up with it and pick a Repair option. If you have only a restore disc, you had better have your data already backed up, because a restore will return everything to as it was when you first bought it. And that means you will need to reinstall all your applications, adjust your settings, etc.
Tom