8300 keyboard and mouse stops working after bootup

My D-8300 was working without a flaw before I left for dinner. Upon returning I noticed nothing was running. The monitor was working but the tower was entirely dead. The psu was hot and neither fan was operating. My first thought was the ps went out as the mobo light was still working, but all other lights were off. I bought a new power supply and installed it being careful about being grounded.

After installing the power supply everything worked great as all fans worked, the front and back lights (all green) were working properly, the Dell logo appeared, the XP logo, even the login password was entered, the desktop appeared, several programs loaded....and then the keyboard and mouse quit.  I had to power down using the front button and restarted. Again, the same thing happened, only this time the keyboard and mouse quit working sooner. This happened several times until the XP logo froze.

I have rechecked all connections. Re-seated the memory, unplugged everything except the keyboard and mouse. Restarted and the same thing happened. Tried starting using F12, got to the page but couldn't respond with the keyboard.

Unplugged the unit, keyboard and mouse, rechecked the connections, held the front button down for 30 seconds, removed the battery and let it sit for 15 minutes, reinstalled the battery, plugged in the unit, but when it rebooted to the bios page, I couldn't input anything because the keyboard/mouse would not work.

Any suggestions as to my next attempt?

Thanks

TJ

PS. June 30. 2008:  I have also tried another keyboard/mouse with the same results.