In both of your cases, the only things I can think of is this:
Rearrange your drives, so that an HDD is the primary master, CD as a primary slave, hdd as a secondary master, cd as the secondary slave, and try new IDE cables too.
I was thinking those reg keys too, because that had happened to me too, but, if neither of you have that problem then....
Other than that, as a last resort, update the mobo's bios if you can, and try another IDE controller if you have access to one, or updated IDE drivers.