I am wondering if you are loading too many programs, filling your RAM and it's just crashing out and Virtual memory isn't kicking in.
Printable View
Can you check your event viewer in vista, See if any critical errors have been logged etc.. Type 'Event Viewer' into your start menu search option
Kernal-Power (This is critical) - thats your problem
Have you put anything new into the computer such as a graphics card? - Your CPU is not getting enough power..
Does the restart happen after bringing back the computer from sleep? etc let me knoww!! :)
EDIT: You might just be best off re-installing windows vista. There are too many probable causes for the problem ;/
I don't think its the extension lead problem becos it wouldn't restart the power would be cut auto and the fuse would blow.. Also i think i would have more problems than you :D!
http://i47.tinypic.com/2mr9l3p.jpg
Thanks to everyone who helped. :)
I'll reformat my PC if the issue persists.