Please follow this simple troubleshooting procedure, courtesy of MS MVP
Carey Frisch:
1. Download and install the WGA Diagnostic Tool
http://go.microsoft.com/fwlink/?linkid=56062
2. After running the WGA Diagnostic Tool, click on the "Windows" tab and
then click on "Copy to Clipboard".
3. Next, visit the following website and create a post in the "WGA
Validation Problems" forum and paste the results of the WGA Diagnostic
Data in a detailed post:
http://forums.microsoft.com/Genuine/default.aspx?SiteID=25
4. A WGA troubleshooting specialist will analyze the data and recommend
an appropriate solution.