Computer turned on, but did not boot up

Hello,

I had a new computer and I had this recurring issue.

There are times when I turned on my computer, the fan and power led turns on, i can open my dvd drive but computer is not booting up and monitor is off.

There are times when I press the restart button, my computer will boot up and work very good.

What could be the problem?

My motherboard has a built in Video so I use it for the mean time.

Please help.

Thanks.