This is how U do it in WinXP ... I dunno anything about XP Pro, but I would guess that it is the same.
Start > Settings > Control Panel > Hardware > Device Manager. Then expand Sound, Video and Game Controllers section and then double click on Video Codecs. In the window, browse over to Properties and it lists all the codecs that U have installed. U can double click on any 1 to see its properties.
Hope this helps U ... Usually when Windows Media Player runs across a problem with a codec, it tries to download it from the internet.
Good Luck