As an example, on my Athlon 2800+ I played an Xvid in WMP and VLC. Couldn't really see much difference in Task Manager - both were bumping along the bottom.
So I tried on my Athlon Thunderbird 1GHz media PC. It went from an average of about 40% CPU usage with WMP to... 5% CPU usage with VLC!!!
So I tried it on a Pentium-II 350MHz machine. This machine can't normally play DivX/Xvid. It played, full screen, at 40% CPU usage! Incredible! It didn't manage to play properly with image post-processing turned up to maximum, however. Not that I can really notice a difference between it on, and it off (the default in VLC).
So, if you have a slow PC but want to watch a MPEG-4 video, and don't want to worry about the hassle of video codecs, download VideoLAN VLC media player now!