Why is there so much lag on Halo PC Trial campaign?

I have the trial version of halo (pc) and there is a lot of lag on the campaign part of the game (the multiplayer is fine)? I know most people have problems with multiplayer, but I have problems with single player. Does anyone know a good way to prevent the lag? I am playing on a laptop pentium dual core T4200 2.00ghz and Intel GMA 4500MHD Dynamic Video Memory Technology 5.0. Thanks for the replies!