Encoders for windows xp?

I downloaded a movie from demonoid on to my NEW LAPTOP........ Win XP Home Edition. Tried to play it on VCL player and Windows Media player and there is no video but the audio works. Do I need an Encoder....Codecs....Decoder....????? Available links to websites would help... I have watched many downloaded videos on my old laptop with the same operating system, never needed anything before!! HELP!!??