Please try the following:
Click on the blue "Change product key" link in the Windows activation section of your System Properties panel. You can access the System Properties panel by right-clicking on Computer in the Start menu and choosing Properties. Enter your product key when prompted to do so, and press Next.
If you get an error code please post it here.
If this activation attempt succeeds, and you're still having trouble validating post the results of the WGA diagnostics tool (http://go.microsoft.com/fwlink/?linkid=52012), so I can help you troubleshoot further. Also, please include validation failure web page error code. It will be gray and in brackets at the end of the message on the page that says validation failed. (It'll look something like this: [0xC004C000]).
Thanks!