Microsoft windows registration

Well I would like to sell my old computer on EBAY, but there is one problem. I had to reinstall windows on it and my wife's old boss let me put it on their. However after 30 days we had to regester it and when we tried it had already been registered on to many computers. Its been well over 30 days but I have not used it in a long time.
I want to erase the harddrive and sell the computer, but would like it to have a working operation system.
Is their any way around the regestering. If I could use windows I could just erase my personnel document and sell the thing. I dont think I will get much for it if it does not work.
thanks

You have a PM.

If you indeed have a legit license then you need to call the Microsoft registration service line. They give you the number when you say that you want to register over the phone. The techs will reactivate your license for one or two installs. It's happened to me a bunch of times... I reformat a lot.