Primary mistake all first time system builders make is not performing a bench boot.
This establishes that the MB/ CPU / Memory is not an issue.
Remove the MB from the case, place it on the box the MB arrived in. Attach only 1 memory stick, KB/mouse, power supply (do not forget the 4 pin connector for the CPU and on video card if exists) and monitor. Attempt boot, ignoring errors.
If system fails to boot, try, in order....
re-seating the CPU/HSF
clearing the CMOS
if exists, swap video card
swap memory and try different memory slots.
If the MB boots make sure the MB stand-offs are in correctly.
If they are not then you have shorted the MB preventing system from booting.
Install all memory and attempt another bench boot before returning the MB to the case, Connect only monitor, KB/mouse and attempt boot. If system boots attach the rest of your components. Except, if exists, a memory card reader. For some reason if this is attached Windows will be given/assigned the last drive letter during install.
If system fails to boot disconnect all components but one, attempt boot. If boot succeeds continue attaching one device per boot till boot failure. This will be the failed device.
In very rare cases, two that I personally know of, it is the Computer case that is preventing the system from booting.
I just finished building my custom computer with parts from newegg (total cost was $1555. The power supply and fans seem turn on when I press the power buttom on my case, but I get no image display on any monitor I plug in (and the monitor lights remains orange instead of green).
So....could this be a problem with the processor? A problem with the video card? A broken motherboard? Please let me know.