Restart your computer. When it is in the command prompt at the beginning loading up your settings, does it register?
(Does it detect a second cd rom drive, either under PRIMARY SLAVE / PRIMARY MASTER / SECONDARY SLAVE / SECONDARY MASTER)
Go into Start, Settings, Control Panel, System. From there, try and find your device manager tab. Windows Xp has it under the tab of hardware. Now, look at DVD/CD Rom Drives. are there any yellow exclamation marks by any drives (this indicated a conflict.) Maybe check to see what it is.
Another thing, if you have recently taken your pc apart or whatever, it is possible that your IDE cable connecting your computer with your cd-rom became unattached. Maybe check it again.
Your cd-rom drive could be disabled under your bios/cmos settings. but I don't recommend you playing with those if you don't know what you're doing.
lastly, it's possible that your cd-rom drive has died. it happens.
Eat. Sleep. Breathe. Ski.