Thanks. I've checked the log and there might be a problem with one of the following files:
GTA - San Andreas\models\coll\peds.col
GTA - San Andreas\models\fonts.txd
Are they original files, or have they been modded?
If they are original files, check GTA:SA works in single player.
If GTA:SA works in single player then try renaming
GTA - San Andreas\eax.dll
to
GTA - San Andreas\eax_backup.dll
and then launch MTA