I've had a DVD drive in my computer that wasn't connected to the Motherboard because I didn't have a SATA cable. Just today I got one from the IT guy at work and plugged it in, now I have the problem I just described in the title.

Furthermore it only turns on the time I press the power button after plugging in the power cable, never any other time. When it turns on the fans spin, the blue lights all over my computer light up, then in less then a second it stops. I hope I haven't fried anything in the computer particularly since I was pretty proud of how quickly I put it in, I'm usually pretty damn clumsy and nothing ever happened before, and I periodically walked over to my sink and touched the faucet.

If it means anything, there is this green light on my Motherboard over one of the PCI slots in my computer, it's always on.

Also, I've tried pulling the cables out of the back of my DVD drive, same thing still happened.

Here is my shopping list from Newegg and the other pieces of hardware I have installed:

OCZ Platinum Revision 2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory

ASUS M2N-E AM2 NVIDIA nForce 570 Ultra MCP ATX AMD Motherboard

AMD Athlon 64 X2 6000+ Windsor 3.0GHz 2 x 1MB L2 Cache Socket AM2 125W Dual-Core Processor

Western Digital Caviar SE16 WD5000AAKS 500GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive

EVGA 512-P3-N801-AR GeForce 8800 GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card

SAMSUNG Black 22X DVD+R 8X DVD+RW 16X DVD+R DL 22X DVD-R 6X DVD-RW 12X DVD-RAM 16X DVD-ROM 48X CD-R 32X CD-RW 48X CD-ROM 2MB Cache SATA 22X DVD±R DVD Burner with LightScribe

SONY Black 1.44MB 3.5" Internal Floppy Drive Model MPF920 Black

A second 80 GB Hard drive

A wireless network card

And my power supply says,
DC Output: +3.3V +5V +12V1 +12V2 -5V -12V +5Vsb

Make sure you have put all cables in correctly (the right way, etc) and also make sure that you have a sufficient PSU for the power required (wattage).

Yeah how many watts is your PSU?

Is it overheating?

You DID use thermal paste, right?

Problem solved, I took the computer to a shop and someone looked at it right in front of me. When I unplugged the DVD drive I still had the cable plugged into the motherboard, the guy there unplugged it from the motherboard and the computer turned on, I bought another cable right there and that's the end of it. The guy suggested that if there was a short in the cable then it would keep the computer from booting just by being plugged in to the motherboard. The idea of there being a short in a perfectly healthy looking cable seemed absurd to me but I couldn't argue with results.