Managed to get some more info, thanks to some of the guys on a Crestron forum I belong to. Here are the responses to the same question you asked.
1. One that I've seen mentioned on The Green Button
(http://www.thegreenbutton.com/) is called MCE Controller. More info is at http://www.kindel.com/. As far as I know, it's one-way control only at the moment - more reliable than simple IR control, but no feedback data, so if you're looking at putting status/song/artist info on a touchpanel you're out of luck.
2. I have the HP Windows XP Media Center on my system
and it is all IR controlled as far the Media Center is concerned.
3. Also checkout: www.streamzap.com - USB IR Receiver plugs into compurter and remote gives you transport controls, number buttons, menu controls, channel & volume, mute & power and 4 programmable buttons to start any programs you like.
Let me know which one you go for and how you get on. I'm just about to get my crestron system up and running and may go down the MS Media Centre myself.