Dont know if its been posted..you can try
Lowering your XP settings
Right click>My Computer>Properties>Advanced>Performance>Settings (Leave it on Best Performance)
Or try to lower XP resolution settings, instead of 32 bit, leave it on 16 bit
Graphic card settings
On some settings, theres ''application control'', for me, this causes alot of lag, might work if u ''uncheck it'' or dont allow it, if possible - set it to performance, or off..depending on your gx card & setting
GTA-SA Settings...
Mess around with the settings, instead of 32bit, try 16 bit..it might not 'look' as good, but if it comes to look & the game running smooth, i rather it run smooth.. & try not to have alot of stuff running in the background (Processors) depending on your specs, they might be hogging up most of the juice!!!
Just my thoughts*