Playing Halo Problem

[Sliktaggin]

Hey , anyway i recently got a new computer a Gateway AMD 4200+ , 2 GB ram , and a Nvidia Geforce 6100. my question is this, while running halo i seem to have choppy graphics really choppy. when i zoom in using any weapons the image gets distorted horribly . i cant make out what im zooming in on..
im just trying to figure out why its running so bad... i got a new 19' LCD moniter but it just appears bad. even worse than when i used to play halo on my old emachine!! this is what i dont understand? do i need to tweak things? i have everything set to high appearance. i just dont know why its playing so choppy. if anyone could help that would be great THANKS!