Is there a software package available that can serve up a hard-drive internal to a PC and make it available over USB to other USB Master nodes as mass storage?

Ex: take your C: or /dev/hda drive on a PC (let's call the computer PC-A), and run a driver program which makes your C: or /dev/hda drive available to external devices as USB mass storage. When you'd hook up another PC (PC-B) to PC-A via USB, it would detect a USB mass storage device, which is C: or /dev/hda on PC-A.

Is this even possible?

EDIT: I know that there are other ways of making data on a drive available between two different computers (eg. putting PC-A's hdd in a USB-drive-enclosure, or having PC-A make the hdd available via a network share). But I'd like to know if the method that I describe above is even technically possible.