Networks File Server... Not Linux (Yet)

I just hooked up a new PC to the Home cinema and am using the old one as a file server. The disks were already in there so it seemed reasonable.

Its all connected to a gigabit switch with a 21m Network Cable and on first glance(watching about 2 minutes of standard and high definition stuff) all looked good.

However, after watching about 20 minutes of an SD file last night the frame rate dropped fairly badly. It could be the file. I haven't had time to try anything else yet, but it seems unlikely.

The files server is running XP and has an old Meedio installation, but really not much else.

Any idea why this might occur?

Is there anything I can do to optimize the fileserver? I don't really have time to setup the Xubunto system I have seen on here at the moment. I'll probably get round to that later in the year.