Upgrade to Windows 8 on an old PC and use the CD key else where later?

I have an old laptop running Vista, that I don't use anymore. Since the promotional price of upgrading to Windows 8 is very attractive, I would love to get it. But it requires Windows already running on the machine.

Is it possible that I upgrade it on the laptop, get the Windows 8 CD key, uninstall it and use the CD key on a new PC I put together myself?

Forgive me if this question is silly. I have faced this situation before.