Ok. Considering Maya suggests a minimum of 512MB of RAM just to open and use some of the stuff, I would think that you are taxing your system using Mental Ray (a resource intensive ray tracing program) and that your system is basically choking to death. Boost your ram dood, and you shouldn't have any problems.
Dave Baer
Professor of Digital Arts
Digital Media Arts College
Boca Raton, Florida
dbaer@dmac.edu