Why does my hp pavilion entertainment pc not detect my dvd/cd rom drive

go to device manager
(Click on Start and then Control Panel. --> Click on the System and Maintenance link. --> In the System and Maintenance window, click on the Device Manager link near the bottom of the window.)
and see if your drive is listed. if not, then go to HP's support site and try to download new drivers for the the hardware.

Try the MS scan tool.
http://support.microsoft.com/gp/cd_dvd_drive_problems