You shouldnt get into any trouble or have any problems.. I bought a gateway PC about 2 years ago, it came with XP, I just recently built a new PC and used the gateway CD to install XP, I have done the updates on both, the XP veritfication, etc with no problems. You should be able to run the XP that you have on your other systems and on your back up.

I think gates and would rather you do that than pirate a copy and cd key off of the internet. Your better off using your good cdkey than a pirated one.