Hey, I know the laptop sucks for gaming but that performance must give you a high fx quality and a good fps.

However:
1. Collaborate in nVidia control panel, configure the gta_sa.exe to match your needs (but don't do all ultra, kay?)
2. Try running with context menu > run with nvidia performance...
3. Try updating your drivers - important
4. Try closing running programs in effort to give you a boost to virtual memory.
5. Try closing applications that use 3D
6. Try reinstalling GTA without mods, cars - clean installation. If you use disk ,it may be scratched.
7. Try running GTA: San Andreas with Windows XP SP3 compability.
8. Check your laptop for CPU usage, before and during game
9. Don't forget also to check for malware.
These could resolve the problem, surely 90%
If it still does lagging, it's something hardware related.
Peace out
