I keep telling people the best thing to do is to run a search for a forum dedicated to GTA:SA. In general the problem your having has probably already been experienced by someone on the forum with ideas on how to fix it.
Personally all I can advise is check all your drivers,check your version of DX, patch your game.
Not working try a reinstall. Not working try cracking it just for the laugh.
Still not working go find a GTA:SA forum and ask there.
"Its only when you look at ants closely with a magnifying glass on a sunny day that you realise how often they burst into flames"