This is using CrossOver 7:
If you create a Windows 2000 bottle, and install Windows Media Player 9, then Napster, it will run, but you won't be able to sign in, ultimately making the software useless.
The reason the sign-in dialog doesn't show up is because it's trying to use SSL, which doesn't appear to work yet in IE6 in a Windows 2000 bottle. Napster won't install in a win98 bottle.
So here's the steps I did:
1) Create a win98 bottle.
2) Install IE6.
3) Use Winecfg to change the Windows version to Windows 2000.
4) Install WMP9.
5) Install Napster. It should complete just fine.
6) Use Winecfg to change the Windows version back to Windows 98.
The software then works to play music. There's a few visual glitches, sections that use IE only partially work sometimes or don't work others and burning doesn't work at all because it relies on a service that it installs, but you can search for and play music just fine.