COD4 playing really rubbish

[AFKMatrix]

Ok I have a Macbook Pro and have installed windows XP on it and am trying to play COD4 and its playing terrible. I have put all the graphics to low and my FPS is 40 and its jerking around like no bodies business. I assume I am lagging but then I can play Teamfortress 2 with no problems and not lag/jerking. So is this to do with my connection as its via wifi and i am downstairs and the router is upstairs?

Its really annoying as its basically unplayable.

[design-is]

Have you tried playing upstairs? Surely that would answer your question immediately.