windows - Assign custom keys to keyboard media buttons

I've a Microsoft Wireless 800 keyboard that has media keys such as Play/Pause, Volume Up/Down/Mute, etc. But it doesn't have any for Previous/Next song/video/whatever. I'm looking to map a couple of standard keys (Scroll lock and Pause Break in particular) to the above ones.

Is there an app that lets me do that ? I'd also like the ability to toggle the custom mapping.