Hello flannagn,
To install (or reinstall) an Upgrade, you must first have the older Windows (that is being upgraded) installed. In other words, to to install the Upgrade to Vista, you need to have XP already install.
If your computer originally came with Windows XP, it also came with Windows XP Recovery Software. The Recovery Software came on either on a seperate disk or included on a section of the computer's hard drive. The Windows Recovery Software is
used to reinstall the Windows that originally came with that computer. If you can no longer find the Windows Recovery Software, you can try contacting the Manufacturer for a replacement copy of the Windows Recovery Software.
Thank you,
Darin MS