First open up regedit, to do this go to the start menu,
click "Run", next type in "regedit" and press enter.
Next go to HKEY_CURRENT_USER\Software\Valve\Half-Life\Settings and double click on "Key", change the value that's in the box.
After that go to file than exit.
Then Reboot your machine and you are set.
and use the following CD key...