it's possible to be "un-detected" in a LAN? i'm talking about a small lan (8 computers, 2 switches) , no special server, software....etc (only winxp, maybe sp1 or sp2)

I want, for example to listen an mp3, see a movie....., without beeing detected by programs like netwatcher(pro), in fact i don't want them to see me at all.

sorry for my bad english and for this lame question