Limiting the number of internet explorer browsin windows that can be opened


I manage a cybercafe and i need help pls. Some of my customers open so many browsing windows at a time that eventually the system freezes or crashes. I would love to limit the number of internet explorer browsing windows that they can open. I would like to limit them to a maximum of 5 browsing windows. Any help on how i might do that would be most welcomed. I am on a windows 2000 platform.