PC wont restart if shut down for more than ~5 minutes

Hi:

I find I'm readily able to restart my Dell PC--No difficulty with this.

However, when I choose "turn off" instead of "restart", I have difficulty

turning it back back on via the ON button.

What is most peculiar here, is that I have no such difficulty if I press

the ON button within ~5 minutes after it shut down. If I wait any longer,

I'll see expected green lights on the kb and on the front of the PC tower.

I'll hear the sounds from the PC showing that it's in fact ON. But nothing

appears on the monitor.

I can get the PC back on by holding the button long enough to reshut it ,

and then pressing it again. Or if that proves not to work readily, I can

simply unplug the power source and press again. It'll turn on.

When back on , it functions as usual.

Why the problem with turning it back on after shut down for more than ~5

minutes? Why does "Restart" work smoothly, but "shut Down" doesn't ?

TIA

-Eli

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Dell PC 640 Meg RAM

Windows XP Professional Edition

Zone Alarm Security suite 6.1.744.001