Check out
Multiscreener for Windows.
It is freeware. The original was for Mac, but it seems an unsupported version for Windows has been released since I last visited :
"MultiScreener (updated June 29, 2009)
Free Multi-Screen Playback
MultiScreener is a set of freeware applications that synchronize the playback of Quicktime movies on multiple computers, using a local network to tie them all together. (It can also sync multiple movies on the same computer using multiple monitors.)..."
Thomas Leong