windows media player doesn’t play video

I am running Windows XP Pro SP2 and using Media player 10 on two networked computers.
On the old backup machine all video files play fine. On the much newer machine many, but not all mpg, avi,asf and mpeg files won’t play. Even the same files that the second machine plays OK won’t play on the newer machine.
All Media Player settings are identical on both machines.
Sometimes the sound is audible with a frozen video frame showing but not always.
All mpg files from my camera are OK on both machines. The trouble seems to be mainly with downloaded files.
Can anyone help?