Steps:

1. Click start in the bottom left corner, then click run.
2. 
   Type 'Regedit' in the white text field, and click OK. This should open the Registry Editor.
3. Navigate to the following registry key.
   • Hkey_local_machine\Software\Microsoft\WindowsNT\Current Version\WPAEvents
4. 
   Right click 'OOBETimer', in that registry key that you just navigated to and then click 'Modify'.
5. Change one or more digits to anything random.
6. Click start and then click run.
7. Paste '%systemroot%\system32\oobe\msoobe.exe /a', in the white text field,.
8. 
   Select 'Yes, I want to telephone a customer service representative to activate Windows', then click next.
9. Click Change Product key.
10. 
    Type the new product key into the small white text fields and then click update.
11. Check that you get a message similar to 'You have successfully activated your copy of Windows.', if you do, restart your computer, and you're done!

Tips:

• To navigate in the registry, click the small '+' buttons left of the words.
• Buy a genuine product key if yours is not genuine.

Warnings:

• This will not work when the installation source and the product key are representative of varying licenses. For example, if the installation CD is a retail license, but the product key is for an OEM license.
• Do not use any product keys that you find on any websites, these could cause you to fail the windows genuine advantage test as those keys are probable already one Microsoft's product key blacklists. Failing this test loses your operating systems genuineness, restricts updates and displays annoying messages to buy a real product key.
• Editing the registry (or any system file) is like surgery: BE CAREFUL! Don't edit anything except what this article tells you to when you open, 'Regedit'. Doing so can cause system instability, changes in functionality, or worse, be unable to use Windows altogether.
• Never tell anybody your product key.