Do you get a small box that comes up and has a countdown to the shutdown or does it just switch off?
If the later is true then it is likely to be an overheating problem.

Yes it is the later it does just shut down with no countdown so if it is over heating do you know how i can solve this problem. Many thanks. this

Yes it is the later it does just shut down with no countdown so if it is over heating do you know how i can solve this problem. Many thanks.

Well I don't know for certain it is overheating but, to me, that would seem the most likely suspect!

Can you see the fans? If so are they spinning freely? (not clogged up with dust)
If you cant see them, do they sound normal?
Does the PC have a lot of room? (make sure there is about 15 - 20 cm between it and a wall)
Do you know if the computer has been over clocked at all?

=)

I will check the fans. We bought the pc from new i dont know what clocked up means. But thanks ever so much for your help.

if you bought the pc from new and you dont know what over clocking is then it won't be overclocked. Overclocking is basically increasing the current to your processor to gain more performance.

How old is the computer?