Day of Defeat

[RincewindTheWiz]

For my version (counterstrike retail) it's HKEY_CURRENT_USER\Software\Valve\CounterStrike\Set tings\Key, for Half-Life itself it will probably be HKEY_CURRENT_USER\Software\Valve\Half-Life\Settings\Key. Try deleting that key and the game will probably ask to enter it again next time you run it.

Oh I almost forgot, to access this key, click on "Run..." in the start menu and enter regedit.exe and press enter, then just follow the key like described above.