cd rom not read

I have 2 cdroms installed. The BIOS recognises them, Windows 2000 shows them
in My Computer and windows explorer, I can eject them from right click wind
ows explorer but they won't read cds. I keep getting the message insert disk
into cd.
That said, sometimes through fiddling with uninstalling drivers on of them w
ill read a cd but when I turn off the machine it's back to NO READ. I changed the cables, the cmos battery and enabled DMA. I have reinstalled win2000. Please could someone tell me what's going on
UPDATE
If I have a cd in the drive/s when I turn the machine on they are recognised and the drives will read the cds. The drives work perfectly after that. If I don't have cds in the drive/s the same problem above occurs.
Weird Eh!!