To remove a program that was installed on your computer, you can go to Start > Control Panel > Add or Remove Programs (Windows XP) or Programs and Features (Windows 7) > select the application and hit Uninstall. If you have problems completing the action on this way, you can use a third party tool like Revo Uninstaller to help you. This tool is specially designed to remove any application by force.
To prevent other programs to install without your permission, you can turn the UAC(User Account Control) ON. You can use Google in order to find a tutorial for this depending on the OS you are currently using.