This may work for you. I accept no responsibility if it does not mind :)
Basically, you can use Microsoft's own Windows Activation Wizard to enter a valid key:
1) Backup the registry, as you will be modifying it manually
2) Create a restore point
3) Go into the registry (Start/Run)
Enter Regedit
Locate the key HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\Current Version\WPAEvents
In the right pane, right-click OOBETimer, and then click Modify.
Change at least one digit of this value to deactivate Windows
4) Run Windows Activation Wizard (Start/Run as before)
Enter %systemroot%\system32\oobe\msoobe.exe /a
From the activation window select Yes I want to telephone....etc
Select change product key
Enter your new key and click Update.
5) If you are returned to the previous window, click Remind me later, and then restart the computer.
6) Repeat steps 4 to verify that Windows is activated. You should receive the following message:
Windows is already activated. Click OK to exit.
7) Click OK.