Your probably missing the codecs needed to play them.

CODECs list for Movies/Alternate players / Video help forums/ video-mp3 fixers

Try using VLC media player, it has the codecs built in and will play most file types.

For WMP try K-Litecode Codec Pack or XP Codec Pack 2.0.6.

If your OS is Vista: Vista Codec Package v4.4.8 (freeware).

__________________
If it ain't broke, fix it till it is!