Gray screen in windows movie maker 2

Hi
Can anyone please help me, I'm at desperation point!
Suddenly I am no longer able to see .avi movies using
Windows Media Player 9, or Windows Movie Maker 2. I am
using an xp pc, have updated all the codecs, but whenever
I try to play a movie, or import one in to MM2, I just
get a gray screen playing throughout.
Sound is fine.
The .avi plays perfectly in WMP9 on another pc.
The .avi plays perfectly in Quicktime.
It's not a .mov file, and it's not corrupt.
This happens with all .avi files.
I know it must be a codec problem, but don't know how to
solve it!
It's also happening using Realplayer.
I've done windows update, have scoured the internet, but
am not sure what to do!
So, when I try to make projects in MM2, I get a gray
video, no sound.
Can someone please help me, I'll be forever in your debt!
At the moment, I cannot use MM2 as I can't see the clips
to edit them!!