Multiple User accounts

I have a multimedia PC in the livingroom that I prettymuch have setup already. However I would like to change everything about it when logged into it as the user account "Media PC". I would like to have it basically so that whoever is using it would only be able to log into as this and only do one thing... Watch movies on it using myHTPC. Can anyone here tell me what I need to do to limit all of the users abilities except to run just one program and not be able to change any files?
Thanks
Duck

hit "Start + R" buttons same time and type: lusrmgr.msc and hit enter
from there you can control who is Guest, user, super user and administrator
In the Administrative Tools you can add more restrictions of Accessing CDs, Floppy disks and other system settings! then to top it for every program you can specify who can access it by right clicking then into properties then specify who can read, write and exec the files