Is it php < 4.3.7 ? If so I would recommend upgrading to the latest, as you can use the GD2 instead - inagecreatetruecolor is preferred and gives better images.
Check that your extension dir is set correctly in the php.ini and points to /path/to/php/dlls as well.
It doesn't have to be in the winnt dir for it to work...
Just seen you have 4.3.7.... GD was dropped in favour of GD2 (4.3.2)... so you'll have to make sure that you are referring to GD2.dll file...