Computer will not start.

Before I strip it down in the morning to go through all the checks, this is what has happened and I am looking for ideas.
I built a computer for my nephew, specs as follows;
Silverstone SuGo2 case,
AsRock 945GCM-S mobo,
Cosair 2 X 1GB pc6400 memory
Core to Duo 6670 CPU,
Asus 4870 graphics card,
Hitachi 500Gb hard drive, Sony DVDRW,
Novatech 550W PSU.
As it is a small case I replaced all the fans 3 X Xigmatec 80mm, and a Zalman CNPS 8700 LED HSF.
I have only just got the Zalman fan and had to take the motherboard out to fit a bracket to retain the HSF, then when I rebuilt it and plugged it in nothing happened.
I have an Antec PSU tester and checked the PSU and that was OK.
Before I replaced the HSF it was running with no problems, and W7 pro was installed and up to date.
It sounds like a duff motherboard but has any one else any ideas.

Reseat RAM and video card. Double check 4 pin power inserted.

I have checked all the connections, and also checked the RAM and Graphics card were correctly seated.

trackrat said:

I have checked all the connections, and also checked the RAM and Graphics card were correctly seated.

You were messing with the CPU heatsink, ya?

Did you check if the CPU itself is correctly seated and locked into place?
Did you check if the heatsink is correctly mounted and locked?

This would not be the first time that I've seen pressure on the heatsink dislodge a CPU from it's socket...

Just took it all apart, and the Mobo is dead.
I cannot see any obvious marks that could point to a short.
As the mobo is only 8 weeks old and I brought it from a local computer dealer, it is going back.