Desk-top PC, running Windows XP, AMD Athalon II, about 6 months old.
Before I take it back to where I got it, anyone got any ideas?
when you say 6 months old, is the PC 6 months old or that you have had it 6 months? I ask because XP is no longer supported mainstream by Microsoft, since 2009, and the Athlon II series processors only came out in 2009 and so few were shipped with XP as OEM software.
Anyway have a look in Control Panel (Classic view) - Administrative Tools - Event Viewer
In there you will find the system trapped events that are logged for Applications, System & Security, most will be flagged with an 'i' in a speech bubble with Information after it, these are 99% OK some however will be a white X in a red circle with Error after it to show err.. Errors. Have a look at these from around the time of your reboot. there should also be at the time of the reboot some events titled EventLog to show when the eventlog was stopped and started. this is the time frame you need to start checking to see what the PC was complaining about before the shutdown.
Also, is the PC set to stop on error or reboot? to see that again look in Control Panel (Classic view) - System.
System Properties window, click on the Advanced tab.
Startup and Recovery, click on the Settings.
Startup and Recovery window, uncheck Automatically restart.
Click OK and again to exit.
Now any BSOD or halt your PC will not automatically reboot.
Rebooting is now manual after a crash.
And, what service pack are you running? Control Panel (Classic view) - System.
It will tell you on the System tab
i.e. Windows XP build 123.567 (Service Pack 3) something like that
Let's see if that helps get you started.