[[SP]Badman]

It's not the problem that SA-MP is not coded for Windows 8, I have windows 8 and there are no problems because Windows 8 uses the same compatibility options as Windows 7, so if GTA works properly on Windows 7, so it should work properly on Windows 8, so basically, you have some problems with your game.

Try going in your GTA SA User files and delete the gta_sa.set file. (It restores your settings) and try again, if it doesn't work, reply.