Struggling with a Dinosaur

Can you boot to the bios screen? Can you try a different monitor? It would be a long shot to have bothe the 6600 and the on-board bad at the same time. When you start the system up does the HDD light continue to be lit and flicker? May be the display is bad?

hmm maybe its full with dust when was last time any1 clean it, old pc usually heat up more, plus 5 years of dust and busted fans,
Have you open and look inside?? last old pc i open had 1/2 inch of dust, really (chipset fan was cloged, cpu had 100rmp, it would shut down in few mins du to overheat)

-I can get into Bios yes.
-Same thing happens on multiple monitors.
-The case is decent and it has decent cooling for the year. I dusted it all out first thing.
-I believe it boots to windows, but without a signal to the monitor. The keyboard lights go through what appears to be the correct power cycles. I'm not certain, because once its done, I can not get the other lights to come on, such as caps lock, or turn off num lock.

Could it be the PSU? If it were failing on boot, I don't think it would just cut power to some of the system. Also the 6600 GT is now completely outside of the case. The PSU is a very good quality Antec from that era.

Could it be the motherboard, or RAM? These seem the most likely as motherboard problems can be pretty finicky, but I don't know how to check that, as I don't have any spare boards with that socket. RAM would likely cause a blue screen, not a video error.

Could it be the CPU? Perhaps it overheats and the PC locks?

What do you guys think?

One suggestion would try a live Linux CD, and see how it runs. If it's locking up, then it is a hardware issue.

Do you have more than one stick of ram? Could you try one and then the other?

Actually it is possible that a PSU can provide enough power except not enough for video. I would discount that for now though. I would not suspect the CPU for now either. I would guess motherboard, cheap ones can fry in less than 3 years.

It sounds as if the hardware is working OK but that there is something wrong with the Windows installation. I assume that you have Windows XP. You will need to boot into recovery console from the Windows installation CD and run CHKDSK /f/p

This will fix 75% of Windows start up errors like you describe

I agree with pjmelect. What are you using to boot it? If it is a hard drive from another system, the Windows installation on it will be missing the correct drivers, and have a lot of bad ones. When changing motherboards, a fresh installation of Windows is almost always required, or at the least, a repair installation (which may refuse to activate unless you beg Microsquishy).
Some older Antec PSUs were notorious for developing the bad capacitor problem, but the way your machine does actually boot makes this seem much more like a software issue.

-Using same motherboard and hard drive as always.
-Still trying to find an XP disk, though I'm not hopeful since I can't even get into the Windows 7 installation.

Oh, if the drive has Windows 7 on it, you'd need a Windows 7 installation CD, not XP; pjmelect said he assumed you had XP.