I assume, you're running Win7? If so, starforce should be the problem...
You can try:
*remove your install completely
*reinstall from DVD - !!Don't start the game!!
*Update to Version 1.1.0.0
*Install Python, Wxpython and the Autoupdater from here
*Go to Autoupdater -> Tweaks -> GTL-Patch (170MB to download)
* Run the Autoupdater (be patient, 15GB skins and tracks to download via P2P)
* When the Updater has finished, you can unlock all cars and tracks in the autoupdater (make TG2001.DYN) and set the car prices to 1 - not 0, as you can't sell them afterwards, if you like to
*Join us on track..
Hope this helps
Georg
Edit: rdjango was faster - I just translated the mentioned FAQ... great work of him!