The VLC media player is an open source and my favorite media player is out with a new version.
It is a highly portable multimedia player, encoder and streamer that supports many audio and video codecs and file formats (MPEG-1, MPEG-2, MPEG-4, DivX, mp3, ogg, …) as well as DVDs, VCDs and various streaming protocols. It is able to stream over networks and to transcode multimedia files and save them into various different formats.
It can also be used as a server to stream in unicast or multicast in IPv4 or IPv6 on a high-bandwidth network.
=> Microsoft Windows
=> Mac OS X
=> Windows CE
=> Sun Solaris
Highly recommend even if you are using Windows XP.