Now that the source code for SlimRoku is available (thanks again TheEndless for that) I decided to have a quick look at why SlimRoku is not working with SqueezeCenter 7.
I tried it against SqueezeCenter 7.0 and I got the problem where I could see the albums etc. but when I tried to play the music it would just say 'buffering' but would not actually play any music - so problem confirmed.
I then tried it against SqueezeCenter 7.3.3 (I think this is the current version) and it worked Took a few seconds to actually start playing (though I think this is because I was doing transcoding etc.) but it definitely played music!
I run my Squeezeenter on Ubuntu, but it should be the same on Windows.
So, if you've had no luck with SlimRoku and SqueezeCenter, try out 7.3.3 - hopefully it'll work for you too