Huh???
I don't quite follow you, but I see two possibilities as to what this could be:
1. Virtual PC's 'Capture Disc' function. This allows Virtual PC to use CDs and DVDs that are mounted on your Mac. It also works for #2, which is...
2. Disk Images. These are essentially virtual disks that can be created from just about anything - a folder on your hard disk, a real (data) CD or DVD, a folder on an external hard drive; all of these are valid sources for disk images. When mounted, they behave just like a real disk, except that they reside on your hard disk or on an external drive you happen to have connected. It's a shame that disk images aren't as easily achieved on Windows as they are on Mac OS X...