Any ideas?

I have a Dimension 3100 with XP home. First issue was no video at all so I ordered a new system board and put in pc. Now I have video, but cannot get pc to even attempt to load windows or boot from cd. It will post, I can get into the bios, I can F12 and select boot menu, I can boot to the utility partition of the HDD. Ran diagnostics from there, everything came back pass/good. When attempting to boot from cd, I get the classic " press any key to boot from cd ", press enter, screen goes blank, although monitor light stays green.

I've replaced system board with another from Dell, same issue.
I've taken out bios battery, same issue.
disconnected cd-roms, tried to boot, same issue.
Removed RAM, & only used 1 stick at a time, same issue
Verified SB jumpers are the same on old & new SB
Tried known good HD, same issue
None of the indicator/fault lights are showing any codes.

What could it be that I am missing to get this pc to boot to either the CD-ROM or HDD????

Issue has been resolved. Both RAM sticks had gone bad. Assuming they were toasted by System Board. Very strange error, but I guess stranger things have happened.