Windows Media Player tends to be very resource intensive and may not do very well on older hardware. There are plenty of other good media players out there:
Wplay, MusicMatch, J.River Media Jukebox, Sonic, C-4, Xaudio, Breathe, Nad, MPFree, Soritong, KJofol, Digideck, Yo!MPEG, AudioActive, Virtuosa, VTT, SCMPX, Soundream, UnReal, Jet-Audio, Philips MPEGPLAYER, WinPlay3, museARC, Maplay, freeAMP, Resonate, NovaStation, UltraPlayer, EarJam, WisMP3, etc. etc. etc.
Here is a web page that has links to all of these: