When the PC first boot (after a while of not using it, wasn't even hooked up for a few weeks), the boot went through to the windows boot screen and then it just shut off.

I unpluged everything from the board to trouble shoot, this is what I was left with:

MoBo: ECS 945P-A (V1.1)
GPU: PNY GeForce 6600 (PCIe)
RAM: 1GB DDR2 (512MB x2)
PSU: OCZ 360w 12v/15A

The next boot, the PC made it through the hardware check, then came a beep and a message. The error read "Checksum Bad press F1 to enter Setup" and then the PC just shut off (I didn't even have time to enter the setup). If I try to reboot the PC right away it won't even make it to the boot screen (the PC shuts off at the GPU bios screen). If I let the PC sit for a good while though it will again boot all the way to the windows boot screen and then shut off.

Any ideas?

Is that the ENTIRE error message? Common issues are wrong settings in the BIOS or hardware configuration error. Try removing the RAM and cleaning it with a pencil eraser then reinsert fully. Be sure to exercise proper static precautions and unplug the computer.

When I saw "Checksum Bad", the 1st thing I thought was a bad CMOS battery...but that shouldn't force the PC to shutdown. Is the CPU cooling fan plugged in & working properly? heatsink securely in place? The shutdown *may* be caused by an overheating CPU.

CPU, heatsink and fan ok. Haven't changed the battery yet but discovered something very odd to me. To see things better I laid the PC down on it's side and it boot all the way into safe mode stayed on no problems at all. As soon as I tipped it up the pc mayed a click sound and shut off.

Sounds like your heatsink is loose. Or you may have a bare wire that is shorting out when in the upright position.

WOW! After multiple checks, I finaly did find out that the heatsink was loose, thank you guys. I mean just one of the 4 pegs was just slightly loose and what a big difference it made, lol.

Thanks again.

Glad to help.