Take a look at "Windows Steady State" to administer the shared PC.
I'm assuming the PC is standalone with different users as opposed to being on a domain / connected to a server.
Windows Steady State is highly configurable on a per user basis, but you may have to think outside the square to prevent a user from downloading software (software installation prevention for a user appears fairly straight forward though) or streaming. For example you may have to restrict access to those sites where the user downloads from. You can also set Windows Steady State to revert all the settings back after a user has logged off. This alone will frustrate them because any changes they may make will not be there when they return after logging off.
In any case do a little bit of research, you may find this is just what you need:http://www.microsoft.com/windows/products/winfamily/sharedaccess/default.mspx
May the force be with you.