unable to load mpeg-2 or mpeg-4 in powerpoint

I am trying to put some short movies (10-30 secs) into Powerpoint (Office XP on a fast machine) for an important presentation.
Mpeg1’s seem to work fine.
Mpeg 2’s and 4’s complain about error loading codec. (even when I try running in Windows Media Player without PP)
I would like to get the best resolution on the projection screen without overly inflated files which are slow to load.
ANY advice would be appreciated.
thanks
JH