WMV & MPEG files sound no pictures

[Hopeless]

I have Windows XP SP2 , when I open WMV & Mpeg files I only get the sound no video. It doesn't matter what program I use.  I have Windows Media Player 11 and Nero showtime.  I am a novice at a computer so please could you keep answers very simple.
Many thanks
Hopeless

[Broni]

Probably some codecs missing. Get them here: http://www.afterdawn.com/software/video_software/codecs_and_filters/xp_codec_pack.cfm