The registry key documented in that article is what applies if the current user hasn't got an overriding setting in their own part of the registry; normally, Windows users have settings in HKEY_CURRENT_USER\Environment that override the system default one (and typically points at %USERPROFILE%\Local Settings\Temp), so it's not that the temporary space is on C:, it's wherever you store your user data (which defaults to C: but you can configure that to lie elsewhere).

[ Note that this registry setting applies only when you log in, so if you change it you generally have to log out and back in again. ]

By the way, when GBW exits uncleanly (and possibly under other circumstances), it doesn't always completely clean up all the temporary files it creates so the *first* thing to do isn't to exit that registry key, it's to pay attention to your temporary file storage under the above path and clean it out; directories that look like "gho32467" where the numbers are random are created by GBW, and if you've got a lot of those sitting around you might want to manually remove them.