Ok, mr. Mars. Here is how you do the trick. You enter My Computer, and go to GTA IV. I will provide screenshots. You need to get to the GTA IV folder. When you scroll down, you will find an .exe file, called "Unninstall_IVMP.exe". Uninstall IV:MP and reinstall it. You should find the IV:MP install pack rar in the Downloads folder. I have made screenshots in a Word File. :P(No time for tinypic, sorry).
I hope you'll get back in game.