Thanks for your suggestions:
To answer your questions:
The laptop is reasonably new (14 months), is connected to the mains 95% of the time (but the problem occurs with or witout the battery in) and I haven't installed any dodgy freeware etc. but I have updated Windows.
I am discounting the theory I had about temperature because i can see the fan kicks in when required and the temp never goes above 68/70C.
I have maintained a log of the shutdowns - it is completely erratic so unlikely to be a worm or other type of malicious software. I did notice that it seemed to be occuring more frequently when using IE, but then I'm almost always using IE.
It also goes off while I'm typing something lengthy or using the PC "intensenly".
So I've narrowed it down to 3 possibilities:
1. Windows s/w problem - 'cause it's telling me on occasions that some files are corrupt
2. RAM problem because I sometimes see a quick flash of a memory dump just before power goes off
3. The problem as mentioned by
OzPax1 which, if you do a Google, seems to have affected many a 6100SP owner in the past - and that's the last thing I want to deal with..
So at the moment it is sat on my desk gavering dust, the hard-disk is in my desktop and I will leave to another day when I have more time to look into it further.
Cheers
Charles