My XP SP2 said I had to re-activate after changing the filesystem of one of my drives form FAT32 to NTFS. Nice feature. I have an original XP licence. My key would not re-activate, so I tried a crack to keep working. After 3 days I got the endless loop: Log in, Say I have to reactivate, get the screen with only the OK-button and the message ít's already activated, click OK and log out...
Even safe mode didn't work.

http://images.thetechguide.com/forum/public/style_emoticons/<#EMO_DIR#>/sad.gif\' class=\'bbc_emoticon\' alt=\'

\' />
FOUND THE SOLUTION!Press F8 while starting, but choose
save mode with command prompt. It will let you in. Start up regedit and change the HKEY_LOCAL_MACHINE/Software/Microsoft/WindowsNT/Currentversion/WPAEvents key.
Reboot and the normal reactivation screen will appear.
Then I dialed the Micro[censored] number and I had to enter the 8x6 code by phone. It couldn't be verified and I was immediately connected to a helpdesk officer who gave me an activationid within two minutes...
I wanted to avoid to call Micro[censored], but it dit cost me a lot of frustration, time and trouble. The call took only 5 minutes totally on a toll-free number.
Good luck.