Error C00D1199 or ""Error: Unsupported Format" means little to nothing in
terms of a single fix and doesnt "always" really mean that you are trying to
play an upsupported format.  There is not a single thing you can do to fix
it.  It will greatly depend on what you are doing when you get the error
along with your system configuration.

One of the first things you will want to check is if you are playing a
supported file type.  The link below links the file types playable and not
playable in Windows Media Player.
http://support.microsoft.com/default.aspx?kbid=316992&product=w98

You might receive this for an AVI if you don't have the correct codec for
that AVI installed.  If you get this while playing an AVI see below...

Using a DirectShow decoding filter such as FFDShow will decrease the
problems you could run into with codec problems.  FFDshow will decode XviD,
DivX 3/4/5, 3IVX, and has support for many other codec's.  It can be
downloaded at http://tinyurl.com/2snnx. (Download ffdshow-20020617.exe and
then run it's installer)

If you just wish to install the individual codec's for your specific video
or FFDShow doesn't support the codec for you file please do the following.
- Download and use GSpot (http://tinyurl.com/328cp)
- Run the installer, open GSpot, goto File>>Open.
- Look for the 4CC code for the file's Video Codec
- http://tinyurl.com/262go will help you.
(Or post the information back here and I can help)

If you get this while try to play MPEG-2 video or a DVD then you will want
to do the following...
- Reinstall your DVD/MPEG-2 decoder
- Update your video card drivers

Some hardware-created ASF's will produce this error.  The fix for that is at
http://tinyurl.com/byv0

In some cases people have run into this error when the wrong audio device is
selected in WMP.
- Tools>>Options>>Devices>>Speakers>>Properties will allow you to change it.

If you get the error while trying to play MIDI's please see
http://tinyurl.com/23vqj

Again, since this error means little to nothing if the above doesn't help
please provide more information in your post.  http://tinyurl.com/2o4h9 will
help you know what kind of information will be helpful to your peer's so
they can try and help.