Dell Dimension Black Screen Of Death

[Boring Larry]

It's a Dell Dimension 8400, and, without any hardware, software, or any other changes all of a sudden, like overnight, it will not go beyond the choices available using F8 to get into the start up choices menu, it also will not boot using a bootable cd such as acronis back-up software or mem test, or windows disc, or dell re-installation disc and will not boot from a floppy, either.
It also takes an extremely long time to respond to keyboard stokes, although in BIOS keys work as normal; 2 keyboards tried there, and at every step, and also should add that on the screen that does show up, it says no keyboard present, although it does work enough to get into BIOS and the start-up menu screen.
After you choose any of the options, it goes to the Black Screen Of Death, however, after watching said process way to many times, you can see where once or twice the BSOD kinda flickers to a slighty less black for just a moment, before returning to it's morbid dead self.
I have:
checked things in BIOS, everything appears good there, all drives read present and accounted for...
played with the boot order any way possible...
disconnected everything to try to isolate something....
checked memory by plugging one stick at a time into all slots; BIOS beep happens only when there is none plugged in
looked for burn marks or any other evidence of electrical mal-function; none visible...
changed motherboard battery...
set things to default, jumped the BIOS, and set things back the way they were before and after each of those steps...
I haven't:
checked the hard drive on another machine, but i have felt it, and heard all other drives, working, at least mechanically....
checked the memory in another machine, as i feel it's good...

Bottom line is, everything acts as if it is working, but it ain't, so, my questions are:
Has the motherboard taken a partial dump?, and
Is there some sort of virus/malware that could take out an entire system like this? and
Does anyone have any other solutions?
Thanks in advance, folks!

[Rik from RCE]

The 2 most common problems on that range of machine are power supply faults and blown capacitors on the motherboard. You really need a power supply tester to test the power supply with which can be bought cheaply on ebay. You also need to check all the capicators on the motherboard to see if any are domed and/or leaking. Any you find will need to be replaced.

Hope that helps!

[Boring Larry]

checked the PS, it's ok, can't see any obvious capacitor problems...I've been reading how over heating is a problem with this type of computer, going to check to see if the goop on the heat sink needs re-gooping, and to see if the heatsink is properly seated...didn't feel loose when this all first started...

[Rik from RCE]

How exactly did you test the PSU?

[Boring Larry]

How exactly did you test the PSU?

I tested it by using a PS tester I bought, and I also tried a known working PS in it...same results...

[Rik from RCE]

On the back of your computer, just above the audio sockets, there should be 4 lights numbered 1 to 4. I need to know what happens with them!