Acer aspire one won't log on

Hello,
I didn't know soon enough about the Alt+F10 buton feature, so i formated my netbook the usual way i did with my desktop. So now the case is that after my first log on
when the system reaches the welcome screen i get a popup screen that says "This copy of Windows must be activated before you can log on." and won't let me log on although the serial # is valid and the windows is already activated. Neither microsoft or acer can provide hel in this matter. Any idea is considered helpful.
Thanx