IE and Vista can't find video codec
 

Hi All,
I put a link to a client's TV spot on a web site that I created for them. It has worked fine for a year or so with IE6 and Firefox on W2k, XP, and most Vista computers.
Yesterday, I pulled it up on a relative's VistaSP1 computer and got the message that it couldn't be played because the installation was missing a codec or it was created with an unknown codec (words to that effect). If I "save target as", it plays in Media Player just fine.
This is a standard def spot compressed to .wmv in Sorenson Squeeze 4.5. The link worked and played ok on my home W2K edit box (I was so paranoid, I had to check it again).

Does anyone have an idea whats going on here?
TIA for any help on this.

Ken