PC Crashing

Hi,
My PC keeps crashing. It does it all the time on power up (when windows xp starts) and I have to keep starting it again. After each restart it goes a bit further until eventually it is running OK. If when I start I go into BIOS and leave it for a few minutes and then start windows, it is usually OK. However, it still crashes randomly once running but normally only when its not in use or has IE7 running. When running any games, etc all is fine. When it crashes it powers down immediately (fans keep going) and the Hard Drive activity light is on. The reset does not work so I have to kill it at the mains to restart it. I’ve changed the power supply and memory with no change.