Although a clean install is often the best policy, some may prefer to go the upgrade route and install Windows Vista over an existing Windows XP system. This procedure has the virtue of (mostly) preserving old settings. However, all may not be satisfactory after this type of installation and you may want your XP system back.
How to do this is explained in this Microsoft article.
Of course, you should have made a complete backup of your previous system before installing Vista and restoring that backup may be easier than the process given in the article. One thing to remember is that Vista changes the boot process so you may need to restore the boot sector as outlined in the Microsoft article. Also whatever backup method that was used should include the hidden boot files boot.ini and Ntldr. A complete disk image is my preferred backup method.