PC to Denon DM39DAB via WiFi?

You could use an Apple Airport Express http://www.apple.com/airportexpress/#airplay and connect to the optical in. Have been thinking about buying the same setup.

Or for £20 more get an Apple TV and enjoy the same Airplay features whilst purchasing the video side of things as well.  That said, the Denon doesn't look to have WiFi built in, so there might be a preference for the Airport Express as you could use it as a bridge to your WiFi router (if a wired connection isn't possible).