There are a few ways to accomplish this.
1)Use a UPNP compliant server, such as TVersity, Mediatomb, or WMP11, all of which are free. They allow devices such as an XBox, PS3, or standalone player, like a Roku Player, connected to your television to receive any media files streamed from the UPNP server, including movies, music, pictures, and in some cases web content such as Youtube.
2)Use a standalone media server box, such as the ones apple makes for media playback connected directly to a TV.
3)Use the output from the PC to connect to the TV. This just requires a video and sound card with outputs, and the proper cables to connect them tot he television. The most popular will be S-Video or DVI/HDMI, plus RCA cords for the sound. Windows can then be configured to use the TV as a 2nd monitor to display the media.