%SystemRoot%\system32\restore\rstrui.exe

If you prefer not to mess around with the System Registry yourself, there's a free utility called RemoveWGA 1.2 available for download on the Internet from Firewall Leak Tester.

Removing WGA: step by step

1. In the Add or Remove Programs Control Panel, turn on the "Show Updates" check box at the top.

2. Open the Folder Options Control Panel. Click the View tab. Remove the check, if any, beside "Hide extensions for known file types." While you're at it, click the radio button beside "Show hidden files and folders" and uncheck the box beside "Hide protected operating system files." Click OK. (Note: If children or computer novices use your computer, you'll want to reverse these steps later.)