Computer will not start after normal shut-down

[yerryd]

My desktop computer info: Windows XP Home
                                        Pentium 4, 3.0 Ghz
                                        40 Gig HD
                                         1 Gig RAM
                                        AVG 7.0 anti-virus
                                        SpyBot Search and Destroy
                                        AdAware

Computer works fine, using most any application. However, when we shut the system down
we are not able to restart for many, many hours (like overnight)!

We have tried to restart immediately, in a couple of hours, or 4 hours and nothing happens.
The next morning, the computer will start right up!

When the computer does not start, we do not hear the customary "beep" associated with the
completion of POST, nor do we hear any other beeps,  see lights on the optical mouse or screen
raster.

I have replaced the CMOS battery.

Please, any ideas? All help appreciated

Thanks

[Backdated]

Duff PSU capacitors possibly casused by overloading. Substitute PSU to test.

[yerryd]

Substituted a known good power supply and everything works great!

Thanks for your help.

[pcdoc4christ]

Yerry D:

You might also consider using a power supply with greater wattage (for example, 400), especially if you have added new hardware.  Otherwise, this power supply might suffer the same failure.

Doc