PC can't bootup

A couple of things to check.

In BIOS setup, are the CPU and RAM speeds correct?

Is the CPU actually supported by your motherboard?
(Intel makes several versions of many P4's that have different Front Side Bus speeds.)

Change the motherboard connectors that fans are plugged into.
Some fans generate too much noise on the Tach out line and cause problems for some motherboards.

Do you have, and know how to use a voltmeter?

If so, check the case power switch. The resistance should be less than 5 ohms with the switch closed.

Next, check the 5 volt standby power line to the motherboard.
It should be between 4.9 and 5.1 VDC.

Monitor the power on and power good signal lines from the motherboard to the power supply.
Does the power on line change states when the power button is pressed?
If not, the problem is either the power switch or the motherboard.

Another thing to check is your plug-in cards.
Remove them one at a time and see if the PC starts powering up correctly.
I have seen modems, NIC, and sound cards that caused power problems with some motherboards.

Chas

open the system check weather the cpu fan is working
if it works it shows power is being supplied to motherboard check weather u get a beep
if u don't get a beep if might be a problem with smps change the smps and check if the same problem presist it will be problem with the mother board

Who cares what is wrong. It is only one day old. If you open it up you may screw up the warranty. Just take it back.

Are you the only one using this computer? Could someone else be turning it off with the little switch on the back of the power supply? If not maybe you should get your house wiring checked

If it is attempting to start then dying during or shortly after the POST (pwr on self test) it may be the power supply is too small. I would take it back if that's possible. It really does sound like a power supply issue.