Hi,
Ring MS they will give you another key, you can add the new key without an install via http://support.microsoft.com/kb/328874

>I hit my limit on my XP Pro installs and it's telling me I used up my registration keys. What do I do? I can get a new XP Pro from Ebay for about $130, but can I install it over the current installation? I have lots of stuff installed, utilities, etc and didn't check the activation because I didn't have an internet connection at the time. I know I'm not the first to run into this. Thanks!