You need an S-Video cable, if the TV you have is analog. Though, digital TVs also have the S-Video port (most).
You may even need a DVI or DVI to HDMI cable. Basically, the PowerBook will treat your TV set, whether analog or digital, as a secondary monitor to which you can use the TV (believe it or not) as an extended desktop (external monitor) or mirrored desktop mode.
Of which both modes will let you watch your movies from your PowerBook.
Please see this thread to which I detail how to do all of this with S-Video:
[http://discussions.apple.com/message.jspa?messageID=7103842]