Just manually remove (delete) the GTA:SA folder and all its containing, aswell the folder "GTA San Andreas user files" in 'my documents', and then reinstall it.
I always keep a copy of my GTA SA on the computer. The game I play, and a RAR file of the game, so when something messes up, I just open that RAR file, and add/remove the files I need to.
Hope I helped...