I took it out of the set-top-box, and attempted to use it in my PC.
I can see the HD in the control panel, but it is not visible from "My Computer".

I attempted to format the drive....can't.
I made it the only HDD in the PC, and attempted to do a fresh install of XP on it, no dice. It just sits and does nothing.

The HDD works when I put it back in the original set-top-box.

To me it sounds like Samsung locked this drive (the box it came out of was a Samsung unit).

Do you think there's any way to unlock this dirve, and use it in the XBOX?

try useing a program called atapwd.exe when you have it in your pc and set as master drive it will tell you if its locked.also you could stick it in a modded xbox and try formatting it with slayers or x-disk