Help needed!!!!!!!!!

I have an intermittent crashing problem. It roughly happens when I start to
stretch the machine doing this like video conversion AVI to MPG etc. Or
alternatively it sometimes crashes when left alone with no programs running.
It may be after 10 minutes or 5 days when left with nothing running but is
usually 10% into converting a video file 700Mb.



Everything usually freezes and a red band appears at the top of the screen.



I have recently installed a second HD and thought this may be the cause of
the problem. However, it still happens when I disconnect the second drive.
Just in case I have just installed a 500 watt PSU.



System

Athlon XP 2000+

512Mb DDR

80Gb HDD

120Gb HDD

DVD

DVDR

NVIDIA GeForce2 MX/MX400

Windows XP 5.1 build 2600

500 watt PSU



Any help would be appreciated. I don't really want to replace every
component.



HELP