Faint Windows loading screen and system hang

Morning!

in preperation for my new build I swapped a few parts about from my kids pc to my old/their new one yesterday. This involved adding some RAM and putting their HDD in the chassis (not connected).
Upon rebooting the windows logo appeared really faint, as if the contrast was massively out, and the blue bar didn't move and the pc just stayed there.
I returned the pc to original spec and tried again...same effect, tried adding/removing RAM 1 at a time..same effect, tried different video card...same effect, tried kids HDD..same effect, tried diff PSU..same effect.

Can anybody ofer some helpful advice? I have a few bits on my HDD I need to recoup!

Thanks

Need your system specs, but it does sound like your RAM might not have enough voltage. Click the last link in my sig.

Thanks for the reply.
From memory, sytem is...
P4d800-x
P4 3.2
1 Gb DDR RAM (can't remember speed)
6600XT
HDD is a Barracuda 80 Gb.

The sytem worked fine that morning, and no BIOS settings etc had been changed.