Create an Chrome shortcut, with spotify as the home page. Then you can lock it down by using host rules, like in this link.
http://superuser.com/questions/322062/can-i-lock-a-user-to-one-site-only-in-chrome
You add that Chrome shortcut to the 'startup' folder, normally located in the start menu.
Then you can make everyone log on as a guest account. You can assign and remove access where you would like to, by right clicking on the drive/folder/document/application, whatever it may be, click properties, then security tab. You can restrict or add permissions there.