Computer turns on then off.

Hello guys, I bought a computer for my newphew which is going to be homebuilt because he is a gamer and will get more bang for his buck. I have built my own computer before so I figured I wouldn't run into any issues, but fate has planned otherwise.

Problem:

Got everything built and installed, turn on the power button and noticed that the cpu fan struggled to spin, then started spinning normally, then completely did a full stop.

At first, I thought I had something wrong with my cpu fan, but I hooked it up to my cpu and it spinned perfectly fine. I thought great, thats out of the way.. So what else could it be?

Then I thought it might be the mobo or PSU, but my mobo lights up fine. Then I noticed when I turned on my computer, that my power LED light powers up for about 3 seconds, then shuts down.

As it turns out, I didn't put the stand offs in place as my old rig had them already preinstalled so I didn't think about it. Well I put on the standoffs and thought for sure it would work now.

Well everything pretty much went the same. Power comes on for 3 seconds (via LED light), fan spins, and then it just shuts off.

Does anyone know what could be causing this? I"m suppose to deliever this computer to my newphew Saturday and it would really suck if I told him I don't have a computer for him. I"m thinking its a faulty mobo, but i'm really just not sure.

Things I tried:

Reseting Cmos
Using both fan headers.
Placing down standoffs (stupid to not put them on my part)

I get no beeps, no post sounds.

Also, when putting in the power swtch pins, they are 2 headed pins which don't tell me which is positive or negative. (I"m assuming this doesn't matter?) Which might be the case.

But before I try that, I want your guys thoughts / opinions on the matter.

Thanks.