What exactly happends when u start samp or singel player? Do u crash to deskop and get a error report?
Heres some stuff that might help without knowing more :
1. Do you have mods installed?
If yes, re-install GTA: SA and don't use mods. can make conflicts/also allways take backup when moding stuff

2. Reinstall SA-MP.
may not be the case
3. Your resolution/graphic settings might be set wrong.
this happend to me and what i did was changing the compability to Windows XP Service Pack 2 or even Windows 2000 then changed it back to Windows XP Service Pack 3, unclicked the box right above the Compability menu Applied the changes and it worked. To change resolution ingame open up Singel player change them there quit and open samp