XP Pro Ugrade License question

Hi
I have a copy of Win XP Pro upgrade that I had initially installed on a test machine. This machine recently died, and rather than fix it, we decided to get a new one. I want to use the XP Pro upgrade to install it on another machine. Is this possible or am I stranded with a worthless copy of Win XP pro upgrade.

I will call microsoft but i wanted to get the heads up before i called
any info is appreciated
thanks