After many research and test, i've found the way to remove DRM from WMV files on Windows 7 Pro.
It's not a classic way, but that works !
i'll try to explain my solution (with my basic english).
First step, go on
http://www.microsoft.com/windows/virtual-pc/ and follow the instructions to install a virtual machine with xp. (It's free.)
Second step, download windows media player 11 and install on your virtual machine
Third step, install a software of your choice (
http://undrm.info/remove-DRM-protection/)
Final step, enjoy !
Note : The network between the virtual machine and your computer is automatically created, each hard drive is shared, there is no configuration to do.