Deskpro EN Hard Drive Problem I am stumped.

I have a Compaq Desktop model Deskpro EN that I hardly ever use. It sits
running. I only have one visual basic billing program that I
periodically use. This morning I went to boot it and got an error
message that the file windows 2000 root\system32\ntoskrnl.exe was
corrupted and needed to be reinstalled. I booted off of the Win2K CD and
tried to repair the disk with no luck. I believe and I/O error message
accompanied this. So i decided to take the pc apart to clean out all
dust, make sure all cables are seated, no dust in the contacts, etc and
then went to reboot. Well now, I get no image on the monitor and the CD
sounds like it is spinning up and getting warm but all I get is 2 beeps.
I have retraced my steps, thinking that I reassembled the cables
incorrectly but I do not think so. I am not sure. There is a separate
cable going to the HD, the CDR and the floppy drive. is this correct.
Then each gets power and the CDR gets some smaller connectors. i am
stumped. I am more of a Mac person so I do not really understand how PCs
go together as well. I do know that the cable that goes from the board
to the floppy is of a unique size and cannot have been switched by
accident. That leaves the CDR and Hard Drive. If anyone can figure out
what I may have screwed up, please let me know. BTW, I had a couple of
spare HD lying around and tried both of them with the same 2 beeps. I
just need to get it running so I can contact Western Digital and see
where I go from here. Please help if you can.