or just read this....
Note that you need a valid serial number which you can buy at your local Microsoft dealer
1. Run regedit and go to: HKey_Local_MachineSoftwareMicrosoft WindowsNTCurrent VersionWPAEvents, on the right double click on "oobetimer" and change at least one digit of this value to deactivate windows.
2. Choose run from start menu and type in this command: %systemroot%system32oobemsoobe.exe /a to get the activation screen and go to the second option which is activate by phone.
3. In the new screen choose the option to change product key, and type in the new product key ("different" version only

). Press update
4. Close that window and reboot your pc. After rebooting rerun step 2 for the second time to verify your activation , you should see "windows is already activated" (You can install Windows XP Service Pack 1 now!).