Dimension 8400 freezes on desktop

Hi,

I have recently been experiencing a lot of problem with this system.
It's Dell Dimension 8400 Intel Pentium 4 3.4 GHz with HT tech
My videocard recently failed, but even before, it would freeze up on boot and on desktop.
I just figured it was a software problem.
But after the videocard failed, it became more severe until 2 days ago where it became constant. I reinstalled windows. There was no freeze during formatting, but when it was on windows installation screen, it started again.
I did add 2 GB of RAM to the 2 GB i had on it before. I did play around with the RAM to see if it would make any difference, but there was no difference.
I did touch the heatsink to see if it's overheating but it was cool to touch.
And more details of the problem, it just stops. During Windows loading screen, when it is first booted.
Thereafter, it persistently freezes on desktop screen. It would be working then all of a sudden, freezes. Mouse doesn't move, tried plugging it out and back in; keyboard ctrl+alt+del does not work; and I also tried waiting it out, but no response.

What could be the problem???
It's very frustrating. Seeing as how my videocard is crapped out, could the processor be damaged as well? What kind of hardware problem could cause this?

Thanks in advance