Hi -- Both.
So long as the decryption library (not sure what the windows .dll file is) but once it's installed then Windows Media player will play commercial DVD's.
That's why installing something like WinDVD will work as it has the built in decryption library. When uninstalling so long as you are using VLC or Window Media player WinDVD won't remove the decryption file.
Cheers
jimbo