I suggest you follow these steps:
1. Uninstall GTA:SA
2. Backup your save files in your GTA San Andreas User Files, and User Tracks.
3. Delete this folder.
4. Delete the folder "GTA San Andreas" under "Rockstar Games" in "Program Files" .
5. Restart your computer.
6. Install SA.
7. Run SA WITHOUT your old User Files.
8. If that works, then you must of had a corrupted file in the SA folder. Now you may put back your save game and User Tracks directory as they were, yes, dont be afraid to overwrite.
9.. If it suddenly doesnt work after that, dont use your save file.
Hope it helps.