Do the following:
Uninstall everything that ever belongs to GTA SA, the Rockstar Games folder, the User files, erthing.
Then reinstall it, from the ground up,
install it in the x86-folder (if you have 64bit),
start in compatibility mode for Win XP sp2, save the mode,
limit it to one core (if you're processor has more then one core).
If that doesn't work something with your OS might be fucked.