does it work without mods? if not. it could that your disk/installer is damaged, if you are totally sure that is not the disk/installer then try using an andvanced uninstaller, clean up the registries(look at google to know how to do it, download a patch of the game,or if still does not work with the mods try to contact Rockstar(if your copy is legal)
If anything else fails the las resort might be to format your PC, but I dont recommend to do that for GTA.