computer freezing

I am running xpwindows. And for a long time the computer freezes and I have to press the reset button and it boots back up. I have had the power supply replaced, I today reloaded windows. Since I spend a lot of time at I Won or Gamesville it seems that the freezing happens more often then than when I am at a regular site like this one. However it has frozen up a few minutes after the computer has started. Could this be the motherboard or proccessor or video card? Anyone have experience with this?