No drive letter(s) assigned to the virtual drive(s), or virtual drive(s) disappear

[Copytrooper]

Check in device manager if any drivers are installed for the virtual devices. There should be cdrom.sys, storprop.dll and redbook.sys.
If no drivers are installed, click on install drivers and point to cdrom.inf - or mshdc.inf (for vIDE) - supplied by OS (in hidden Windows\Inf folder).
If all mentioned drivers are installed, but also some additional filter drivers check this thread, too:

http://forum.daemon-tools.cc/f26/my-real-cdrom-dvdrom-drives-missing-440/

Also check if you've a file infcache.1 in Windows\Inf folder, (re-)move it, then try again.