Hi guys…problem here…pc has turned itself off twice in the last hour no warning or error messages…anyone any ideas? 
Suz are you getting any error messages when this happens or does it just shut down and thats it? Does it restart?
No error messages…zilch…just restarts…only done it twice so far…don’t know where to start looking…done all scans… ???
Suszannah, is there anything inside of c:\windows\minidump?
Did you set your energy saver (into screen saver settings of Control Panel) to shutdown your computer after some idle time?
Can you uncheck the ‘automatic restart’ of the computer: Start Menu > Control Panel > System > Advanced > Restart (?)
Do you get a blue screen after doing this?
step one first…what is minidump? sorry to appear dumb on this one… :-[
When the computer crashes, the content of the memory is poured into a file.
This could be analysed and, sometimes, the cause of the error (crash) discovered.
There is full dump (all memory RAM), minidumps…
Follow the Start Menu > Control Panel > System > Advanced > Restart
I’m not sure of the English translations of the commands…
Thank you, will take a look and get back to you… 
It might be heat issues. I would check that out also.
Technical…no blue screen when I unchecked ‘automatic restart’…nothing in minidump either…it hasn’t happened again…yet only the two occasions last night…
darkparrot…pc only 3 months old would I expect to see heat problems already???
So, gremlins, power off failure or whatever 8)
You could Suz. You might want to open you case and make sure all of the fans are working. It might also be a good idea to take a compressor or canned air and blow all of the dust out.
If anyone in your house smokes you want to be careful, I have had to work on smokers computers and have had most the openings blocked by tar.
If doing this doesnt help, I dont know what your problem could be at the moment.
Could the power supply be failing or over heating?
What were you doing on the PC when it turned off?
If it was a black screen, then it is more likely to be a hardware problem, heat (as mentioned, usually CPU), RAM or PSU (power supply unit), in any case (excuse the pun) as this is a new system you could have a repair under warranty.
Hardware issues are difficult to track down and if you try to get a repair under warranty, you ultimately have to talk to techs and try to explain the problem. If this has been a one off occasion it may just have been a blip in power, but if it is a regular occurance, your supplier may be the only option.
There are programs out there to monitor CPU temperature without having to be a techie, a good freeware program is Everest Home Edition, it also provides a comprehensive list of information about your system and components.
Thank you all for your helpful replies…will let it run and see what happens, as DaidR says it is still under warranty and I have taken out an extended one also…will certainly take a look at Everest.
At the times it automatically rebooted I was using the webcam, I thought that may have been the problem…
I had a 300W power supply and experienced sudden reboots. I then changed it for a 560W power supply and it has never mysteriously rebooted since. 
Hi guys…have installed Everest Home Edition as suggested by DavidR…it tell me my cpu temperatures and fan speeds, but how do I know what is the acceptable range? :
Hi Suszannah I had the same problem a few months ago random reboots, no BSOD, no minidump files turned out it was my PSU on the way out. Replaced it , no probs since…
You should be able to do a search for your CPU’s specs (temperatures, etc.) at the manufacturers web site.
What is your CPU and what are the temperatures?
CPU motherboard 31 degrees
Aux 34 degrees
WDC 34 degrees
Fans…CPU…1985 rpm
chassis ........993 rpm
is this the information you wanted?..
The values sound OK.
Could be faulty RAM, short power failures, faulty (or weak) PSU… hard to say, I’m afraid.