1. Install San Andreas as usual.
2. After installation, navigate to the folder you installed SA:MP (default: c:\program files\rockstar games\gta san andreas\)
3. Right click on gtasa.exe and go to "Properties".
4. Click the "Compatibility" tab, change the compatibility option to "Windows 98 / Me", click OK.
5. Download and install SA:MP as normal.
6. Play!
Should work, you know...
Or try something like "2000/XP"...