Possible power supply problem

A friend has a brand new computer which at times reboots over and over again for no apparent reason, often before even starting Windows)  When we brought it back to the store, they saw no such phenomenon.  Could it be the power supply?  (Could it be that the voltage in my friend's house is too low?)  I have a perfectly 450-watt good spare power supply removed from a Pentium 4 computer.  Is it suitable for the new computer?  (If so, I can just replace the power supply and not argue with the store.)

Details:
OS:  Windows XP Pro SP3
MB: G-31/1600 MHz/VGA
CPU: Intel Core E5300/2.6 GHz
HD: 320 GB SATA II

[Moderator]

First let's make sure it's a hardware problem. Go to System Properties - Advanced - Startup & Recovery Settings and uncheck "automatically restart". If the computer continues to reboot it's probably hardware related, otherwise the next time there's a problem you'll get a blue screen error message instead of a reboot. Post that entire message here.

I'm sorry that it took so long for me to reply, but the other party was not immediately available.

First, when I asked her to uncheck "automatically restart," the computer notified her that the setting for the virtual memory was too low and did not accept the automatic restart change.  I had her change the virtual memory settings and the automatic restart was then accepted.

Thereafter, the computer functioned normally for 2 hours and then turned itself off.  This is in contrast to the previous condition, where the computer rebooted.

Now when she starts the computer, it turns itself off almost immediately.

On the basis of the new information what do you think the problem is?

[Moderator]

Well it's certainly a hardware problem. And the change she made to "automatically restart" is not the cause of the computer shutting off - it's a coincidence but unrelated. It works at the store but not his/her house..... have you tried it in your house (or some other 3rd party location)? I'm not a hardware guru, but the obvious possibilities (to me, anyway) are any of the following: power supply, ram, as you suggested - something with the power at the house.

But hang around - someone with more hardware expertise is sure to respond.

[Moderator]

Lowest cost solution...borrow a known good PSU of the same or greater wattage and swap it in there overnite...
You will know soon enough if that's the issue.
If not we need to look elsewhere.

borrow a known good PSU of the same or greater wattage and swap it in there overnite...
You will know soon enough if that's the issue.
If not we need to look elsewhere.

He said he has one.

...I have a perfectly 450-watt good spare power supply removed from a Pentium 4 computer.  Is it suitable for the new computer?  (If so, I can just replace the power supply and not argue with the store.)

Sounds like the right one, give it a try.

Thanks to all for your suggestions.  Replacing the power supply did indeed solve the problem.

I must say that I was a bit hesitant, out of fear that if there had been a serious short somewhere, that coud have destroyed my brand-new power supply and still not have solved the problem.

Fortunately, that was not the case.

very happy to see your problem got solved.i have entered this forum not lonmg, but see your computer problemwas solved, i plan to ask some professional people for help when  there is something wrong with my laptop .