Don't know if this will work with everyone else, but I have fixed my game crashes and slowdowns by updating my motherboard chipset drivers. I had the latest drivers from my motherboard manufacturer so I had to go to the chipset manufacturers page to download the latest versions. Specifically, this updated the AGP drivers which was what I assume fixed my problems after reading on the NVIDIA forums of game, video, etc slowdowns other people experience also with other games. Now this was specific to my video card and motherboard, so it's something I'm throwing out there as something that can be tried if all else has failed.
In reading the NVIDIA forums, peple with SIS chipsets on their motherboards should use the latest NVIDIA drivers as well as updating the chipset drivers.

My specs:
CPU: Athlon XP 2000+
Motherboard: Gigabyte GA-7VAX
MB Chipset: VIA KT400
Video Card: XFX NVidia 6600GT AGP