I don't know what is your configuration but do that:
First, uninstall GP4 correctly
Then, install DirectX 9.0c, excepted if you're running under Vista wich use directX 10 (don't install the game with directx 8.1)
Once it's done, re-install GP4 in this order:
1- GP4 game
2- Official patch 9.6
3- GP4.exe decrypted
5- Launch a quick race, esc, save & quit the game
4- install GPxpatch
Launch the game with GPXpatch...
If all is correct then, you have choice to install new cars, misc and other add-ons manually or with CSM (Zaz tools)...I suggest you to have a look into the GP4 faq ...to find links for new cars, mods, tracks etc, look into the forum (you can use the search function for a speciefied file, car etc..)
And here are links for mods, cars, tracks, misc and other add-ons...
[
www.grandprixgames.org]
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