Computer Freezing

I am having problems with pc crashing and freezing up at random times.
I have windows xp professional installed.
I have done various checks but all come back as ok.
When looking in the event viewer it seems to show a lot of red errors on True vector engine and a program called dhcp, Not sure if they are related to the freezing.
Also i have just installed a new Geforce 4 graphic card. And it only seems to be happening since then, Wondering if thats related to it somehow.
Would appreciate any information as to what may cause this problem.
Thank you in advance.