This is a simple fix...
1.) Run the Activation program for Windows.
2.) Choose "Call a Windows Tech Support Person for assistance"
3.) It will give you your CURRENT Windows ID, and when you call you tell it to them.
4.) They verify your ID, and give you a new 25-digit actication key.
5.) You test it and make sure it works...then you hang up.
I did this last time i had to update registry keys and i had lost mine. Totally free and takes maybe 5 mins.
You might havs screwed Windows by going in the back way and doing some no-crack junk...so i dunno how to help you there. If you have 1 good cd-key already in use and they want a different one, calling them will get your problem fixed. i've now done this on 3 of our home computers wanting new cd-keys for activation and it works 100% of the time.