But for people with OEM license, or upgrade XP to win 7 license, then once you re-install, your COA may not be able to be activated again even when it is genuine. This is because OEM version can be activated only once, retail version multiple times etc.
To solve this, go download ABR or ABRBeta (for window 7) from ABR (Activation Backup and Restore) | directedge.us
Run the backup program and it will generate 2 files and keep the whole folder in a safe place. The next time you reinstall window 7, just run the restore program to put back the activation. Note: You should do this only on the SAME machine with the SAME COA.
This way, you don't have to face the problem of Microsoft message 'your copy of xxxx has been activated too many times'