Laptop display distorted in Windows only

Hi,
I have been troubleshooting a laptop display. Laptop runs XP. I have tested hard drive, memory, CPU fan etc. No matter what I do I get red lines or other distorted areas on the display after a short while running. It also will boot with no display only black after a time and I try to reboot.
I have uninstalled tha nVidia drivers using add remove and device manager. I get the same results when I connect a CRT externally. The only time I do not get the distortion is when I run MS MemTest. It boots to disk and loads and runs for hours with no display issues.
Thanks