Booting DOS from a CD - Invalid System Disk

[jrobot]

Update: Ubuntu still does not boot, and I got the error message again: continuous short beeps = power error. Does this mean that my PSU is completely broken, or could there also be a power wire dangling against something it shouldn't?

I'm going to sort through my PSU cables now.

 

[jrobot]

Update: Ubuntu boots again - think it was the existence/nonexistence of the third HD that caused that problem (I removed it).

However, I added the IDE drive again, and the system does not boot when there is a disk in the drive.