MoviePoster will run on pretty much any PC made in the past few years. It doesn't need to be particularly powerful, or expensive. Many users will re-purpose an old PC they aren't using or purchase a low cost mini pc.
If you are looking for a small, low power PC, the systems listed below or similar would work well.
HP Stream Mini Desktop on Amazon
Quantum Access Windows® 8.1 Mini PC Stick on Amazon
ECS LIVA X on Newegg
Microsoft Windows Vista, Windows7, or Windows 8.1
.Net Framework 4.5 or higher
Direct 3D capable graphics card and driver
Enough hard drive space for the poster/trailer cache. (At least 10GB of free space is recommended)