After upgrading from XP Pro to Vista Business any attempt to play a track downloaded from Napster brought up a pop-up window full of random text with "Media Rights Usage Acquisition" in the title bar. It seems that the licence files get screwed up somehow during the Vista upgrade.

I found this tool - http://support.microsoft.com/kb/916249 - which resets the DRM system. I downloaded and ran it and everything is back to normal now - Vista & Napster is possible!