When installing Windows 7 it's very picky about which installation disc you use (Full vs. Upgrade) and if improperly matched with your key can prevent you from activating your copy of Windows usually returning an "Invalid Product Key" error message. You may also see the same or similar error when you do a full, clean installation with an upgrade disc. This can be very annoying, especially when you have a legitimate key and disc but just didn't install it the way Microsoft thinks you should. In the event that you are having trouble activating your Windows 7 installation with a legitimate key, try the following registry hack:

1. First make sure there are no pending tasks requiring a reboot.This is indicated by an orange shield icon next to your shutdown button on the Start menu or in the notification tray.

2. Open the Registry Editor (Start → search for "regedit" and hit Enter)

3. Navigate to the following registry key:

    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Setup\OOBE

4. Double click on MediaBootInstall in the right pane and change "Value data" to 0

5. Open a command prompt with administrative rights (Start → search for "cmd" and hit Enter)

6. Run the following command to reset Windows activation status: slmgr -rearm

7. Reboot your computer.

8. Run the Activate Windows utility (Start → search for "Activate Windows"), enter your upgrade product key and activate Windows.