Have you ever been able to run the game?
Have you checked that your PC meets the specs?
Have you, after reinstalling, gone into your program fuiles, and deleted the GTA SA folder? (Windows is a bit lazy, and leaves a few files behind)
-> Make sure you delete the SA user files too, as the gta.set holds all your settings, and can sometimes get a bit picky after reinstalls.
Have you used any no-cd or downgrader patches for your game? These replace the .exe file, which is the piece that's failing.