Windows Media Player with Napster DSP files

I have ported all of my music from my old PC to the new one, which includes quite a few tracks that I downloaded (legally) from Napster as DSP encrypted .wmv files. When I attempt to play them on my new Windows 7 PC with Media Player 12, a pop-up appears telling me that "The previous webpage might require the following add-on: 'Napster' from 'Napster'. Click here to allow it to run..."

I click there, select "Run Add-on", and seconds later the same pop appears again. It's really quite frustrating. I have downloaded and installed Napster software on my PC and the tracks play through that player just fine. Any advice, other than using a different player?