horrible lag in and out of cutscenes

[KAMIKAZIKYLE]

i recently bought just cause 2 on steam and im getting horrible lag [nearly unplayable] the first cutscene for example where the chopper gets shot at is so laggy that it takes up to 10 min just to get thru it the video is super low fps and the background noises like the rockets hitting the chopper and the machine gun fireing happen way to early i made this somewhat lesslaggy by lowering everything down to as low as possible but i shouldnt be having this much trouble with it [i can play fallout 3 and newvegas on medium and mass effect 3 on medium and starcraft 2 on high]
here are my pc specs
cpu=AMD Phenom(tm) II N620 Dual-Core Processor
ram= 4 gigs ddr3
gpu= ATI Mobility Radeon HD 4200 Series 320 MB
yes its a laptop but i shouldnt be having these problems i would like to play the game [i have gotten nearly a hour inot the game [just after the car chase] but it is so laggy and low fps its just not fun to play and the cutscenes remind me of a bad kungfu movie]