Question : Problem: BIOS posts, but computer shuts off afterward...Dell Optiplex GX270

I have tried replacing the CPU, BIOS Battery, checked cables for hd/cd-rom, etc..., replaced memory, powersupply.
It does seem to be a motherboard issue now.
I received this error message during boot one time..."Previous attempts at booting this system have failed at checkpoint [PnPo]. Contact Dell Support for help."  Has Does anyone have any info regarding this "checkpoint [PnPo]?

Answer : Problem: BIOS posts, but computer shuts off afterward...Dell Optiplex GX270

It would refer to Plug and Play 0(number zero) This can be referring to the first slot on the board or the agp slot if so equipped.
If you have any cards in slots on the board remove them one at a time and see if the error goes away. If it does then the card that has been removed is probably the culprit.