Go into the Device manager of your computer and see if there are any warning symbols there.
Both PMB and your DVD reader will be detected by Windows as external optical drives in exactly the same way. That tends to suggest a Windoze problem possibly corrution in the drivers.
Either way choose to un-install the drivers then reboot and Windows will auto re-install. That should fix it but you may have to re-install PMB..or MPB or whatever that thing is that Sony gave you!