A normal shutdown through the start menu works perfectly.

For the problematic shutdowns, there is no recorded event suggesting a cause. The computer turns off without warning - exactly like if you removed the power cable without the battery in. It does not restart. To turn on again requires a number of attempts at the power switch.

A faulty voltage regulator sounds quite reasonable. But why would the laptop spontaneously shutdown when using the web and office but not when stressing the machine using a burn in program?! Have you any idea how I would check the voltage regulator and/or replace it? First thing I guess is to find it.

I have the opportunity this weekend to try a fresh install of XP and Office, so I will give it a go. I also have some time now, so I'll fish out the screwdriver.