TOUBLESHOOTING: DXP will not recognize burned DVD-Rs

My firend has a Memorex 4x dual-x1(PioneerA06). The drive seems to burn fine and the burned discs play in stand alone players and through PowerDVD on the PC, but if I go into my computer or Windows Explorer and click on the drive, I get an error saying there is no disk in the drive. At the same time, DVD Shrink will read the burned DVDs from this drive, but XP doesn’t seem to recognize them. Oddly, XP does recognize retail (rented) DVDs. Any thoughts?

Yes, ASPI is installed.

thansk!