I have a 792n HP.  We ere having trouble with it sporadically turning off for no reason.  The Light on the power SUPPLY indicating that the UNIT is plugged in was would go out.  Sometimes it would flicker when you FIDDLED with the cord.  It finally went out all together so I thought it was the power supply.  I put in a new one, but the computer still will not turn on.  No fans, no drives spooling up. Nothing.  I also already tried switching out the cord with a KNOWN good cord.  No joy.  Any suggestions?

THx
AceSo you have changed the power supply and the power lead?

How many sticks of RAM do you have? If you have 2, boot up with each one on its own. If you can, try booting the machine up with a different hard disk - one that you know to be working correctly. Alternatively, try taking your hard disk and booting it in a different machine. Try taking the front panel off and check out the power button switch - make sure it is connecting properly.

It sounds like a motherboard fault, the steps above will rule out problems with the RAM and hard diskI wouldn't swap the hard drives.
Depending on which version of Windows this is there's a good chance it won't work...and it will not tell you if the drive is at fault.

p40ace where did you obtain this PSU and do you have a manuf. name and wattage rating from the sticker on it ? ?IMO i think this is a case of trial and error, would definatly USE the method to rule out ram above and make sure the PSU is connected properly, go over the system again and make sure every cable is pushed in properly, but at a glance i would say its motherboard issue??