ASUS P5N-D Boot Error

I just recently purchased a quad core and an ASUS P5N-D 750i SLi Motherboard. I hooked everything up, booted it up, nothing turns on. SO I go back into my tower, make sure power is plugged in (which it wasn't firmly) and start up again. Now, I'm getting a "DISK BOOT FAILURE, INSERT SYSTEM DISK AND PRESS ENTER". I have two Hard Drives, and the other just shows a black screen. I'm assuming that the HDD with Windows installed is the one with the error. I made sure that all cables are plugged in, which I'm pretty sure they are(They DO show up on the BIOS) and I disabled the floppy drive. Are there any other settings which should be changed? And how can I fix this?

CPU: Intel Q6600 2.4 quad core
Mobo: Asus P5N-D 750i SLi
RAM: 2GB Corsair XM2(1GBx2)
GPU: XFX nVidia 7900 GT/GTO

Everything worked fine before the upgrade...

First of all, if you change critical hardware (motherboard/cpu) without reinstalling Windows, it may crash on you (kernel becomes hardware dependant based on installed drivers).. have you tried going into the recovery console and typing fixmbr or fixboot?

Also, make sure that the hard drive that you have your windows installation on (the disk set as active system) is set as highest priority for hard drives in your bios for booting