If you frequently reinstall Windows and want to save the hassle of reactivating it every time you do that (apart from saving time calling Microsoft after a certain number of activations have been performed), here’s what you need to do:

1) Go to C:\windows\system32 or C:\winnt\system32 (depending on which directory Windows is installed in)
2) Copy wpa.dbl and wpa.bak to a CD or another drive
3) After reinstalling Windows reboot into the Safe Mode by pressing F8 continuously while the computer is starting up
4) Go to C:\windows\system32 or C:\winnt\system32 (depending on where you installed Windows)
5) Rename wpa.dbl and wpa.bak to something else (e.g. wpa.old and wpa.old) in the current folder
6) Replace the wpa.dbl and wpa.bak which you had backed up earlier into this system32 folder
7) Restart the computer in normal mode

The computer should be activated as it was before