I have the same problem like many others!
I bought my windows vista 6 months from todday and i didnt have any issue until today.
I tried to upgrade the windows mobile center (my mistake) and then I am a thief or victim for microsoft because I CANT PASS THE WGA.
I have a valid key that activate my windows copy but It cant be validated.
The big problem is that I cant comunicate with microsoft in my country (Greece) because the office is closed today!
Please send me a note what I can do.