System Crashed - 'Disk Boot Failure' but HDD boots in defferent PC

My PC crashed then would not boot, sending no signal to the monitor event though all fans, lights, hdd etc appears to start running normally.

So I removed all but one stick of RAM, inserted an older gfx card and got it to send a signal to the monitor but it displays the following message - 'DISK BOOT FAILURE, INSERT SYSTEM DISK AND PRESS ENTER'.

I then removed that HDD and replaced it with an older one, but it still displays the same message.

Next I took the original HDD to a different and fully working PC, and lo and behold the PC booted into Windows (Safe Mode). So I have managed to recover my data!

Anyone have ideas as to what may have happened - could it be a motherboard problem, in that the GTX8800 and the HDD are not being recognised?

I also took the GTX8800 card to a different PC but it would not post, though the PSU may be underpowered, but not altogether certain.

Anyone had a similar experience, or could shed any light on this? Need to figure out what exactly I need to replace, if anything, before spending the cash!

I have spent hours trawling forums and seeing many similar threads, but nothing that quite matches my case (specifically the HDD working inside a different PC).

Thanks in advance!

Spec as follows:

Windows 7
Intel Dual Core CPU
Asus P5n32-e sli
Geforce 8800GTX
8GB RAM
500 GB Samsung HDD