Installing many applications requires administrative privileges on the computer you are installing them on. Because of this, standard users who try to download and install such applications on their Windows 7 computers will see a User Account Control (UAC) prompt requiring them to provide admin credentials in order to complete the install. Since standard uses aren't local admins on their computers, they cannot complete the install. This is good news for network admins who don't want users to clutter up their machines with free software downloaded from the Internet.
Per-user applications however are another story since they don't display a UAC prompt when you try and install them. Popular web browsers like Firefox and Chrome are per-user applications and can therefore be installed by standard users on their computers. The best way to prevent users from installing such applications is to use Group Policy to configure AppLocker policies that prevent specified per-user apps from being able to be installed. For more information on using AppLocker to block application installation, see http://www.windowsecurity.com/articles/Securing-Application-Execution-Microsoft-AppLocker.html.