I believe that would involve changing/modifying user permissions.
I don't have any Media Center versions of XP, so I cannot tell you how to do that...but I can provide some reading material.
Which edition of Media Center?
If this were XP Home...I'd simply install the Security Tab and use that to control the new account. The Security Tab allows an admin to define user access to files/drives more precisely.
How Do I See the Security tab in XP Home (Doug Knox) - http://www.dougknox.com/xp/tips/xp_home_sectab.htm
In dealing with permissions, I would advise against using the DENY column for any user. Rather, I would suggest using the ALLOW column and not allowing any privileges to users which you don't want them to have. I can control all the privileges for a drive, a folder, a file...by right-clicking on same.