I try to run this game and every once a while it will work, but most of the time I get this error box to pop up, "prism3d.exe - You cannot run multiple instances of this application at the same time." Anyone who can help me fix this problem would help a lot.

Buy a new game, a better game than 18 wheels of steel

What you need to do is download the latest drivers for your video card, depending on what card you have, you have to go to the MFRs site and download them and install them