You're talking about map mods, right?
You won't be needing a special installing program.
It's a matter of 'Copy 'n' Paste', first you extract the files from your ZIP or RAR file and then you paste the contents into the data folder.
If the files that need installing are packaged inside an installer you open the installer and look for the apropiate directory, by default it should be:
C:\Program Files\Rockstar Games\GTA San Andreas
Hope that helps, I'm good at modding, just crap at explaining