The advice I have, from one I trust, is to use the following set of helpers for QuickTime:

1. Perian. 
Technically a QuickTime plugin. It therefore adds a large number of translators to those already embodied in QT. The current version 1.2.2 is the last that will support PowerPC Macs or any Mac OSX 10.5 or earlier.
<www.perian.org>

2. Flip4Mac WMV <http://goo.gl/RDRGN> and Silverlight <http://goo.gl/16ED>.
Snow Leopard users need to have QT Player 7 installed in addition to the QuickTime Player.app in the Applications folder. I think Silverlight is installed when Flip4Mac is updated.

3. RealPlayer. Still used on many websites. Get the 'Basic Player' version [not the Plus version] from <http://www.real.com/?mode=rp>

For years I found I could not play various videos. Because I tend to 'collect' applications, eventually I had all of these installed when suddenly, I found I could view almost any video which I came across. I did not know why but the above explained it.

The good news is that all these are FREE except for RealPlayer Plus. You need them all.

------------------------
Regards from brianF
===============

On 08/06/2011, at 9:26 AM, Geoffrey Heard wrote:

> At 3:46 PM +0200 6/6/11, Þorvarður Davíðsson wrote:
>> 
>> I use MPlayer OSX Extended to open and play videos, including videos with the ending .wmv. In very rare cases .wmv files cannot be opened with MPlayer OSX Extended for some reason. In such cases I select the file in Finder, Get Info > VLC. From now on this .wmv file will be opened with VLC, all other .wmv files will be opened with MPlayer OSX Extended.
> 
> Thanks for that tip. I had been wondering what to do since Flip4Mac stopped working with the asdvent of the latest QuickTime player.
> 
> Cheers, geoff