Originally posted this question in Software zone, but it could well be a hardware problem. DVD writer can play commercial movie DVDs. It couldn't read a commercial Roxio v. 10 installation DVD, but was able to read a copy created by another PC. Stopped reading DVDs it had previously created. Doesn't seem to be able to read commercial music CDs.
We tried deleting the LowerFilters registry entry (the UpperFilters registry entry didn't exist), uninstalling the DVD writer from within Device Manager and rebooting. The problems above still persisted. Further testing showed that it could burn a music CD from media files on the PC which was playable on another CD player, but it could only recognise the file table itself, whining that it couldn't play that file type or maybe a CODEC was missing.
HP technician suggested backing up our files and reinstalling Vista.
Any other suggestions?