I've been wondering the same thing as I recently moved back to Win 7 from WHS 2011. From what I've gathered a server OS is what will work but I feel is overkill for this type of situation. I also have the HTPC user autologin for MCE and media related while the admin user has a bunch of other automated things running like dropbox with belvedere. I too don't want the HTPC users messing with the admin stuff.
I switched back to Win7 as I am more familiar with it, not used to using virtual machines for servers and was tired of using hacked drivers to try to get things to work (web cams, video capture cards).