install 2 copys 1 for online and 1 for single player
first uninstall sa and delete gta user files in my documents
install sa again
then rename the folder GTA San Andreas to GTA San Andreas mp (in C:\Program Files\Rockstar Games )
then uninstall sa in add and remove programs
then reinstall sa again now in C:\Program Files\Rockstar Games u have 2 folders 1 GTA San Andreas and the other 1 GTA San Andreas mp
from the GTA San Andreas mp folder put a shortcut on desktop for gta_sa on the desktop and rename it to samp
do same again for GTA San Andreas folder and rename gta_sa to gta_sa_sp
now u have to games instaled 1 for single player and 1 for multiplayer
just install mods to the C:\Program Files\Rockstar Games\GTA San Andreas mp folder
this way u can keep single player and mp away from each other