Tom29,
If a nongenuine installation of XP Pro is currently installed and the Certificate of Authenticity is for XP Home or Media Center Edition, you will have to use the Recovery Solution supplied by Compaq to cleanly install XP Home back onto the computer.
If a nongenuine installation of XP Pro is currently installed and the Certificate of Authenticity is for XP Pro, you should be able to genuine-ize the current installation without a reinstallation by using the Product Key Updater utility, which you can access from this page: http://www.microsoft.com/genuine/selfhelp/pkuinstructions.aspx