Your probably missing the proper codecs to play them. This thread has good info: CODECs list for Movies /Alternate players / Video help forums.

I use theXP Codec Pack 1.3.0 for Windows media player. It seems to work pretty well.

VLC media player is a good one that has the codecs built. You may want to use it as a back up player as it will play most files.

Some files are bad/corrupt. Bitzi Web Lookup is a way to help sort out which files are good or not.

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