(Solved) How can I see what file is in use over the home network?

Okay, a little background for this rather strange problem:
I have three systems in the house all networked over ethernet. All are XP Pro 32 bit, SP 3.
This one is Primary which also holds media storage hard drives.
Next are two Secondarys, really dummy terminals that support internet and can stream media from Primary (video and audio).

The problem lies in the fact that there are three users who will all want to be watching a particular video file at the same time but at different start times. To put it bluntly, my neices and wife all want to watch friggin Twilight in their rooms but I only have 1 digital media version on the drive. This invariably leads to the stream skipping and I'm up to my ears in girly anger over why the magic box isn't letting them see Edward properly.

Is there a way to display what files are in use over the network from Primary so I know what file I should not move around/copy at that time?

I know this is an odd question and noone seems to have an answer. Hopefully you folks do.
I know a big solution is to make identical files for each of them but I want to keep this machine at least somewhat legal. /shrug

Stored locally on the video media drive.