Right, we'd already established the problem wasn't with your original PS/2 keyboard. Using a known good USB keyboard resulted in the same Diskette Drive 0 seek failure error as when you plugged another PS/2 keyboard into the green PS/2 port? That error occurs if a floppy drive is malfunctioning or if there's no floppy drive physically installed in a computer but the floppy drive controller is turned on in BIOS. Do you have a floppy drive in your 2400? I don't recall you mentioning whether you were able to access the BIOS at any point during your troubleshooting but if you can go in there and check the Diskette Drive A setting under Drive Configuration. If it's on & you have no floppy drive in the system then turn it off. See if the system will boot up normally & the USB keyboard will work after this.
When you used a USB keyboard did you try plugging it into several different USB ports (including the one you keep your mouse plugged into) after you got the Diskette Drive 0 seek failure error?
Motherboard was the problem and I was able to get a refurbished one from Dell for $129.99
and now the computer is working great again!
Thanks to everyone for all the helpful suggestions, they were much appreciated!
Thanks- I had just thought of this and went online to do it and your suggestion popped up! I already got rid of the old keyboard as I hadn't thought of buying and adapter!