Windows XP Activation Key

I had XP Pro installed on my computer, and recently it has not accepted updates and would not download certain applications. And when I tried to run updates it stated that the Product Key was blocked for some reason.

I used an XP key from another PC that I use and it activated it with no problem, my questions is will that cause a conflict with the computers, and if so what can I do to reverse this use of a Product Key on two computers.