I had my 2 DVD-Drives has disappear after updating Xp with the recent Dot net 3.5 & dot net 2.0 updates.
I was able to fix it by updating the Raid controllers (for the drives) via the Device Manager.
They showed up as exclaimation marks. Right clicked on them & seleceted update (using web)
No RegEdit or anything else required.
Seems the MS update "disconected" them