What you could do is remove (one by one) the memory sticks and using an eraser (like found on a pencil) gently scrub the contacts, wipe the stick clean and replace it.
Also do the same to any pluggable PCB's.
Reseat all the ribbon cables (both ends).
If you have access to another system download the Ultimate Boot CD for Windows, burn it and see if the system boots off of it... (might require you to change the boot order in the BIOS)
The Boot CD has numerous tools you can run to diagnose your hardware...
UBCD for Windows