My first suspicion is that the installation of Real One or another player
might have modified the MCI settings on the system.
PowerPoint doesn't use the Windows Media Player. Instead it uses Window's
MCI player. On Windows 2000 and XP systems the player is called
mplay32.exe. You can test the WMV file by going to Start > Run and typing
mplay32.exe and clicking OK. Then open the WMV file and click on the play
icon. It probably won't run. The reason it doesn't run is the mystery that
has to be solved.
The first step toward diagnosis and correction would be to visit
http://www.soniacoleman.com/Tutorials/PowerPoint/multimedia.htm and read it
all with special attention to section 3. At the end of that section are two
lists of what the MCI settings should equal in the Registry of Win 2000 and
XP systems. Check your Registry against those lists and edit to make them
match. Then test the file again.
Please let us know if that doesn't help.