get the license number from the command line
Recently somebody brought me a system that was soo badly corrupted that I could not get into the window interface to get the license number to re-install. she had the CD provided by compaq but not the manual or number. I tried unsuccessfully poking through an attempt at exporting the registry. After giving up I called compaq with the hope that based on the hw serial number they could help me. He wouldn't do that for me but did tell me how to get the number by myself.
When I got to the command line I typed EDIT USER.EXE and did a search on -OEM- and there it was. Next reboot I used it to re-install