Ah the good 'ol XP.
Try installing the game once again. If the control panel wont remove it for you, than it might think that the game is already removed. Probably some file is missing or the registry is messed up. If you succeed in installing it again, de-install it completely.
After that I would suggest that you carefully study this thread and try the described procedure:
http://www.subsim.com/radioroom/showthread.php?t=133290
After you're done. Install the game once more, and if possible get the UBM add-on so you can enjoy all the latest supermods. UBM add-on can be installed right on top of the v1.0. It will bring your game to v1.5.