DVD/CD device "functional" but illiterate, at our wits end

Running XP Pro, device manager with floppy, cd and combo dvd/cd drives functioning properly but cannot read contents of any floppy, cd or dvd (have tried several). Can see that cd or dvd is in the drive via "My Computer", but cannot read contents nor run executable files. We are able to play audio cd (not sure how as we can find no program such as windows media player or real player running, we just hear the music). This is an intermittent problem as sometimes they work and at other times they do not. We know the disks are good because we are able to view and/or run all of them on our other computer.

Have not loaded any new programs and they were all working before.

What we have tried:
Checked all cables and they are secure.
Disconnected the stand alone cd and made combo the master.
Uninstalling and reinstalling the drivers (several times)
Uninstalled AVG (virus protect) and Zone Alarm (no change)
Loaded optimized defaults in bios (worked a few times, but back to same intermittent problem)

We have read every question on your board related to cd/dvd drives and drivers. We have tried several other sites as well as the most pertinent newsgroups looking for answers and no one will even answer our question. It's probably some silly basic thing that any fool would know, but we just can't figure it out. We will be indebted beyond all if someone could give us some solid advice as what we can do to fix this problem.
TIA
B&H

Try replacing the IDE-cable. If you put on an 80-pin cable, that wont work, it needs to be a 40-pin cable.
Make sure that one drive is set as master, the other one as slave. The red or blue line at the edge of the cable should be near pin 1 on all drives and motherboard.
In your BIOS, check that both IDE-channels are enabled.
Some BIOSes require that where a CD is installed, you set the type to "none". Check the mobo-manual.