On your Windows 7 install, go to "Activate Windows", you can try "Activate over the phone"(can't remember exact message), but it will give you a number to call, and an installation ID.
You will put that installation ID, and then you will either get a robot voice prompt asking "how many computers is it installed on", or you'll be transferred to a real person.
If you get the real person, just tell them what you have done, and that this license of Windows is only on "One" computer.
If you get the robot voice, just tell it one computer and it should let you through.
Unless your license is an OEM copy, where it's only valid on the computer it was installed on, you should be fine.