Any distro provides a good base, but IMHO Slackware gives a better starting point than many as it already includes mp3 support, which many lack. However, like any distribution, for anything more than mp3, ogg, flac and xvid support you will need to get hold of at least the MPlayer win32 codecs, as well as libdvdcss for watching encrypted DVD's. These are available as slackware packages at LinuxPackages
You do not need MPlayer for the win32 codecs, they work fine with Xine, Slackware's default media player. Of course MPlayer is an excellent media player in it's own right.
EDIT Another might alternative is to install Realplayer 10 for linux, although finding the adless version can be a pain, I think they hide it on the HelixPlayer site.