Build hits aggravating road block

How's it going everyone? I started a budget build, buying one piece at a time pretty much, and finally got everything together.

Biostar MCP6P M2+ mobo
AMD Athlon II X3 2.7Ghz CPU
4GB (2x2GB) DDR2 667
eBay Case that came with a 500w ATX P4 PSU (hoping this is the extent of the problem)
500GB hard drive connected via SATA
Random disk drive I had lying around

I turned it on a few days ago (at the time I only had the mobo, CPU, RAM and PSU in and had all of the cases leads hooked up) to make sure I was getting power and had everything hooked up right so far and it immediately came on, all lights and fans working. I got the last few things I needed today and hooked up the disk drive and hard drive. I plugged all my cables in (monitor, keyboard, mouse, power cable) and went to boot it and.....nothing. The mobo doesn't seem to have a light on it to indicate whether its working or not but I noticed that when I turn it on, the power LED on the front panel and the LED in the fan on the side of the case light up for a split second and then immediately cut out. No fan movement and no sound from the hard drive. Not sure why it would have worked fine the other day and now won't do anything. I don't have a PSU to swap out with it right now but am buying one soon. Hopefully it's just cheap eBay crap doing it's usual thing but I wanted to double check that there wasn't anything I'm missing or not looking for. All responses are appreciated, especially hasty ones, as my laptop's hard drive has maybe a week or so of life left in it lol.
 
Have a very careful look around and under the motherboard, for short circuits to the case. The metal stand-offs are very easy to miss out, or fit one where it doesn't need to be. It's obvious, but take out graphic card and re-fit, make sure any buttons or catches are clicking into place. Remove RAM boards, and re-fit. Make sure all power cables are plugged into the motherboard, check motherboard manual to see if extra power plugs are needed by the CPU, (4 pin header type), or graphic card.
 
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