Product Key mayhem

I am in a little predicament.

I tried using my computer this morning and it said my copy of windows 7 ultimate is NOT genuine. Maybe it isn't

I purchased Windows 7 ultimate when it came out and installed in on my home computer and it works great, BUT on my business computer i installed it with a product key that I purchased off of eBay (Paid $100). But its now saying its not valid. I assume the moron sold the key multiple times. I knew I shouldn't have dealt with eBay and just purchase a key from MS.

Live and learn I guess. So here is my question:

Since I purchased the FULL version of Windows 7 Ultimate can I purchase an upgraded windows seven for cheaper and use the key on my existing computer that is currently invalid? Or will I need to downgrade then upgrade again?

Thank you!

You'll most likely have to go buy a new cd-key from MS.

Yep I think I will have to do that...

By the way what happens since my version of Windows 7 is in this non-activated state? Will i walk into my office one day and wont be able to use the computer? Or will it just annoy me till I activate it?

I know I will lose out on updates and be annoyed but is that it?

Wow thanks a lot for your help. Very helpful forum!