are you sure that they are installed correctly. The ribbon cables can be tricky to connect if the drives are already mounted in the case. I have never heard of Windows XP not recognizing a cd-rom drive. Did you check LG's website? That would be a good place to start
Gotta agree with svtcobra, this more than likely 'Isn't' a driver problem, our standard CD-Rom at work is that one, it's not plugged onto a RAID card or anything is it, where the drivers aren't loaded correctly?
Check the cables like Cobra says, make sure they're uniquely ID'd... ie...master, slave etc... i never trust Cable select for anything