I suppose it is possible that mplayer was not upgraded when you upgraded to 8.04. Try opening synaptic package manager, enable the universe and multiverse repositories, reload synaptic, and search for mplayer and reinstall it. Then install (or reinstall) the ubuntu-restricted-extras package that has most of the codecs. Also go to medibuntu https://help.ubuntu.com/community/Medibuntu
and add that to your repositories as per the how-to. Then reload syanptic and install the w32 codecs. You can also get libdvdread and libdvdcss with medibuntu for playing DVDs.
If you use the mplayer plugin for firefox then update that as well.