Chio, that would be something if the WMV files had a bad codec but I wouldn't put anything by Microsoft. For the MPEGs, you most likely need XviD, as the standard MPEG1-2 codecs are already installed by default.
You should pick up a good codec pack, I use K-Lite Codec Pack but you can get another one from
http://www.doom9.org
This could be hardware related as well..
edit: Winamp is absolutely horrible for video files, use as last resort only. Also, windows media player 10 has been out for a whlie, consider downloading it
