Look at the Alcohol 120% "virtual cd/dvd-rom drive emulation" I think it is the "Alc 50%" part of 120%. Deinstall or disable it. Also look in Windows XP services ("Start" -> "Execute" -> "services.msc" as Admin) whether there is sth like the Win XP IMAPI or InCD Server of Nero or sth of Alcohol running. If yes, right-click -> "stop" the service(s). Maybe disable those you dont need permanently (right-click -> "Properties" -> change from "automatic" to "manual" or "disabled").
Maybe this may help. I had InCD Reader (for UDF formatted CDs/DVDs) making DVD-Rs unaccessible on my laptop!