sherlocklgeEver had a video or audio file you could not play?  Most likely yes and the reason is 99% sure a missing codec, a good and easy approach to combat this is to install a codec pack which contain most used codec’es (currently I use Vista Codec Pack which seem to contain all that I need and btw also works fine on XP).

Should you however run into problems with codec’es then you may want to take a look at this “Codec detective” which will assist you in determining just what codec’es you have and their versions, this combined with a little googleing should be of great help.