As already said, you should run everything GTA related in 'Administrator' mode.
Go to your GTA SA Directory and right click the main 'GTA SA' file.
Click properties then compatibility and at the bottom tick 'Run as Administrator' then apply.
Also do the same with the SA:MP Client just to be sure that everything will work.
Hope it works for you, I had problems with GTA after changing from XP to Win7.