As Alvine said, call Microsoft. If you are able to provide sufficient proof of purchase (to their satisfaction), they will be able to help you.
Keep in mind, that an Upgrade license may not activate if it was a fresh install; you needed to do an upgrade from the previous version (even if that version is completely overwritten, e.g. WinXP -> Windows 7). If that's the case, re-install the WinXP, then upgrade it.