To add to the above (seriously check out Flex Components).

Free:

WinDoor does doors and windows

Paid:

WinDoor**+** does doors and windows, not to be confused with the above even though it's from the same developer Window**+** only does windows, from the same developer as the above two

I seriously doubt that these will work on your pre-6 version (as per your profile) of SketchUp.