Clive you should be able to see an error message in the System error log in admin tools, not always very clear but it may help and will be in there now so you dont have to wait until it happens again.
There may be quite a few but if you know roughly about what time it happened that will narrow it down a little, double click on the error and it will open another window with some info that may help to show what the problem is so take a screen shot and post it  

I turned the computer on at around 6.10 am and there are several events relating to drivers not being installed, but this is to be expected in Safe Mode anyway.  There is a reference to a failed attempt to connect to the internet (obviously not possible in safe mode)  The next flurry of activity was at 8.37 when I returned to the computer, touched the mouse, and had an automatic reboot.  I've shown that error message in the next post.

@Clive
Found this on the web, may shed some light on the problem, also bugcheck is started by USB problems more than any other ause

Errors of this type are usually caused by faulty hardware, although software can do the same thing; antivirus products are often the cause.

To check if it's a software problem, perform a clean install, but before installing any software leave the system running. If you don't get the error, it would suggest a software fault.

Try reinstalling your applications one by one. After each install leave the system running for a while. That way, if the machine crashes, you will know which piece of software to blame.

However, it's much more likely that hardware is playing up. Have you overclocked your processor? If so, try dropping the speed back to see if this solves the problem.

If this doesn't change anything, the problem is likely to be with either the memory or the motherboard. Windows XP uses more memory than Windows 98 Again this is going to have to be a case of trial and error. If you can, swap the memory in your machine with RAM from another. If the problems stop on your machine but start on the other, you've got your culprit.

Using a process of elimination it shouldn't take long to track the problem down to a stick of memory. Fortunately, memory is dirt-cheap at the moment, so it's not an expensive problem to fix.

Brian

you little turd turncoat, you said you woz on Moz,   Clive you've let me and Simon down badly  :'(

I use both IE and Mozilla at the same time.  Mozilla is very useful for those dodgy sites which disable the back button you know!  How I hate those.  I'm going to try and force a BSOD tomorrow and get a definitive error message.  When I only get one problem a day - not long after first boot - it's very easy to live with.  Today for example it has been impeccable apart from the 9am event.  I start to panic when, for apparently no reason, it becomes more frequent.  The day it happened 14 times made me think that the end was in sight.

If we are talking about a re-install of Windows, a new motherboard and new RAM, I think I would consider buying a new machine as they are relatively inexpensive these days.  I don't need a top of the range model as all I use it for is WP and Internet.  

I haven't overclocked it Brian, it's a 2200 so it's quite fast enough as it is.  But I think I should start backing everything up in readiness for a major disaster.

Fron a non-techie point of view, I'm starting to wonder if something may be being affected by heating up when you first boot from cold.  I think if it were my PC and I had time on my hands, I might take out,  clean and reseat the Graphics Card, PCI Cards, and Memory Sticks first, and possibly even refit the CPU.

Hi Clive

XP has a learning curve and when it reboots due to a driver load error it will give more time to the system to load drivers thats why it sometimes clears up problems by it self

Brian

Just a thought.  Is there any way I can force it to boot up more slowly on a permanent basis?