If you are trying to update Windows Xp through Windows Update or download software from Microsoft Download Center which is protected by WGA validation, then the solution is "muBlinder" from p2pLife Forums.
Using muBlinder you can easily download updates and protected software like Windows Defender or the latest DirectX runtime environment for your windows.
muBlinder is regularly updated so you won't have to look for a new solution once Microsoft changes something over at Windows Update. A new version is released almost as soon as the previous one stops working.
Before you begin, download and install .NET Framework v2.0 from here, other wise you'll get an application error.Now put muBlinder.exe in a new folder and run it. Go to "Validation" tab. muBlinder will check whether you have the required LegitCheckControl.dll needed to validate your Windows. If you did not install WGA validation tool then the DLL will be absent and you will get this message.
Go to Microsoft Update and install the WGA validation tool. Now the muBlinder will look something like this.
Now press the "Enable" button. The DLL will be replaced with a patched version and Bingo!! you are ready to install updates and download every damn software at Microsoft Download Center. This is how muBlinder will look after you have done so.
Now press the "Go To Microsoft Update" or "Go To Download Center" button. It will open in a new muBlinder window. You will have to keep muBlinder running while downloading updates.
I am putting up muBlinder for download but you will have to register at "p2pLife Forums" to download the updated version once Microsoft tries something else to make the WGA work.