right click the model and click apply existing material and then select lambert 1 (assuming you didnt change lambert 1 (which you should never do any ways as it's te default material for all new objects)
And maya on a 450 is gonna be a little ropey I'm on a 700 and it's sometimes a little iffy but not generally too bad.
Alan
Technical Director - Framestore
Currently working on: Your Highness
IMDB