You could accomplish that very easily using Windows Media
Encoder.  I witnessed the very operation you are hoping
for in action just yesterday afternoon.  A coworker had
taken his PC's TV card and fed the stream into Encoder -
Encoder then made it available on the LAN.

Encoder is configurable so that users on your local lan
can access the stream very easily with the HTTP protocol.

You don't need Windows Media Services to accomplish
this.  But it's fun to play with (when you get it setup
correctly...).

Matt