1. Go to RUN by clicking on the START button, and type REGEDIT. A window will open.
2. Now expand HKEY LOCAL MACHINE on the left hand corner.
3. Then expand SOFTWARE
and then WPAEvents
4. Now on the right side of the windows, double-click on OBETimer
5. Another window will open. In the window, select everything and press delete. Then exit.
6. Go to RUN and type: %systemroot%\system32\oobe\msoobe.exe /a
[there's a space between exe and /a]
7. A windows will open entitled 'Activate Windows'. Choose Yes, I wish to call.
8. Click on Modify Product Key button which appears at the base of this window.
9. Type: THMPV - 77D6F - 94376 - 8HGKG - VRDRQ in the given key space and click on Update. Press 'Remember Later' on the windows that appears next and Restart your windows.
10. After restarting, perform the 6th step again. You'll see a message telling you that your Windows is activated!! :D
This is how i activated my copy of windows and it's still in working condition! Hope it works for you as well :) If it doesn't, leave a comment and i'll attend to it as soon as i can.