I recently had a power supply go bad on an old desktop (custom built) that was used mostly by my girlfriend. Rather than replace the PS, I decided to move the Hard Drive (40GB Western Digital IDE) to another machine (Compaq) I had laying around that had just died from a hard drive failure.

When I power on the Compaq, it goes through the Bios checks and then before loading Windows it gives the error message "Boot Disk Error - Insert System Disk and Press Enter". If I boot from the Windows XP disk, it shows that there are no existing partitions on FOUR available drives. This seems wierd to me because rather than showing no drives at all, it shows four drives and says "Cannot Access Drive" on all four of them.

Also, if I enter the Bios at startup, the hard drive is not listed in any of the IDE connections, but the CDROM is. I switched the IDE cables to make sure that one of the motherboard IDE channels wasn't broken, and the CDROM works in either channel but the HDD works in neither.

The wierdest thing is that if I push ESC to enter the BOOT MENU rather than the BIOS, it shows the CDROM and Western Digital Hard drive clear as day. If the drive was officially dead, why would it be detected in the Boot Menu but not in the BIOS?

I have double checked all jumper settings and they are all correct. I have reset the CMOS and reverted to BIOS defaults. I put another old HDD in the machine so I could update the BIOS, which also did not fix anything. Is it possible that this Compaq machine has some kind of Bios-corrupting virus or something? The HDD worked fine the day before but does not work in this machine.

Somebody tell me I missed something stupid....

jumpers set correctly? could load bios defaults perhaps

Did both.

I would have no problem accepting this hard drive is dead if it would stop giving me little glimmers of hope. Sometimes it pops up in BIOS, most of the time not. Very often the drive shows up in the "Boot Menu" but never boots up.

I wish I knew what was going on..

Hymm, without knowing the exact model of the Compaq and motherboard and the drive model taken from the old machine it is hard to say, but it sounds like a DMI problem or TATTOO problem. Meaning that some software has to be on the hard drive so the bios and other component on the motherboard can interact with it. Different models of Compaq and HP have different tattoo software.
You may want to try:
1. Wiping it out and starting over.
2. Going to Hp’s site to find the DMI software
3. Put the drive into another machine
4. Hook it up as a slave to see if all of the information is there and transfer it to the master

I put the drive as a slave with another HDD, and even still the BIOS did not detect and neither did Windows.

Can you put the PS from the new machine in the old machine and put the hard drive back in the old machine to see if it is still functioning?