If you wanna reinstall your computer and have it on default directory (Program files/Rockstar Games) then you do this
I done it and it was working and it backed up all my windows and old files:
put the cd in your cd/dvd rom. start the installation, just go normally like next next and bla bla, then it restarts (your computer) and then installation of windows appears, wait till it finishes, install some updates, programs, go to C (if thats the hard drive you saved it on) and find windows, under it there should be a folder named windows.old find GTA SA directory and put it in the default folder (c, program files, rockstar games, GTA san andreas)
When you run it it should work with no problems.