Why I would get no video on a board with integrated graphics

Hello -- A friend asked me to investigate her Dimension 2400 after a known piece of malware infected it.
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I did the following, after backing up her data: ... (a) Updated the firmware for the two optical drives. (*see note) . . . (b) Used Western Digital diagnostics to reformat and test her WD800BB hard drive - all was good -- "long" test. . . . (c) The hard drive right now is in freshly formatted FAT32, no OS installed. . . .(d) The board already have latest BIOS previously.
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*note: Before wiping her hard drive clean, I found that I could not update the firmware on one of the optical drives using the Win-based flash utility provided at Dell downloads. It gave me errors and freezes. So I removed that optical drive and put it into a second machine where I was able to successfully flash the firmware. I mention this because it *might* be relevant.
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So now I've got it all back together, and I can't get video. It has integrated 845G video chip. There is a speaker on this board and it signals no errors. Everything fires up the way that it should, but no video.
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I get no keyboard errors, but the keyboard does not seem to be talking to the motherboard as Num Lock doesn't light up, does not respond to F2, nor to Ctrl+Alt+Delete.

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I have cleared CMOS with jumper on more than once. I've removed 3V battery more than once also. Any suggestions with the Dim 2400 much appreciated. Thanks!!

...///((( edit )))/// ... I just fired up this machine with no RAM, and it have no beeps of errors. Either I am wrong about the presence of a motherboard speaker, or ... I dunno!! Thanks

SupremeLaw --
Uhh, the testing without RAM was not about video -- it was about seeing if the motherboard would signal errors of a boot without RAM. (Thus all the verbage I wrote about the onboard speaker.... ... ...)

Anyway, AwsmGy thank you I will double check for some connection problems. It occured to me later to try a different power supply also, just in case one of the voltages is bad on the existing PS.

Very strange... Perhaps your HSF wasn't on correctly or got unseated. The cpu could be dead or the motherboard could of just gone into some error state when it detected the overheating.

I would put the old processor back in with some fresh thermal paste and try it again just to make sure the CPU is dead and it wasn't some other problem. (you may want to reset the bios to defaults and save it before trying the original processor again, in one case this solved a problem I was having)

I saw this website after a yahoo search and it quickly helped me diagnose an issue that I've seen many times when repairing computers.
I always thought it was just a motherboard on the way out.
I reseated the cpu (fused to heatsink) and the onboard video fired right up.
Thanks for the great forum!!!

Sincerely,

Rob