|Another alternative might be to use a software restriction policy in MMC.|
You could list the names of all of the executabes for browsers i.e. iexplore.exe or opera.exe etc to prevent an alternative from being installed and used.
I'm sure this would work to restrict the program files from working, and should be good if the user isn't savvy and doesn't change the executable name.
Use software restriction policies
Using a software restriction policy, you can prevent unwanted programs from running; this includes viruses and Trojan horses, or other software that is known to cause conflicts when installed.
Software restriction policies can also be used on a standalone computer by configuring the local security policy, or can integrate with Group Policy and Active Directory.
Click Start > Run > type GPEDIT.MSC > Go to Computer Configuration > Windows Settings > Security Settings > Software Restriction Policies
Note: This feature is not available on Windows XP Home Edition
P.S. If I remember right, when a software restriction is placed on the pc an alert messages the user that the software has been restricted by the system administrator.