Windows – product uninstall keys located in the Registry under Windows 7 x64

This may be a little bit too obvious but the official Microsoft Support article has two possible methods for changing the product key. The first one is certainly the one you've already tried, but there is also a second method using a VBScript.

If you copy and paste the VBScript given in microsoft article http://support.microsoft.com/kb/328874 into a text editor like notepad, and then save the file with a .vbs extension (use quotation marks around the file name if you find that the .txt extension is automatically appended).

Then execute that script with the new product key as an argument as shown in the instructions given beneath the script. Obviously if you have noted saved it to the root of your C:\ drive the exact command you'll need to type into the run dialog will include the location where you have saved it. Alternatively you can run this from the command prompt in the same way.

msconfig stores its configuration in the registry under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Shared Tools\MSConfig. The sub key startupreg contains information about the (location=) registry startup entries that you disabled, the sub key startupfolder contains information about the (location=) folder startup entries.