When you uninstall a Windows application, the uninstaller doesn't remove all the files and registry entries created by the application. Sometimes the developers were too lazy or they thought those files might be needed if you reinstall the application. The end result is that your computer loads slower because your registry contains more entries and the hard drive stores unnecessary files.
Revo Uninstaller is a software that lets you completely uninstall programs and it's easier to use than the built-in "Add or remove programs". You can search for a program or locate it in a more intuitive way: enter in the Hunter Mode, open the program you want to uninstall and drag the target to your program's window.
After finding the program, Revo Uninstaller analyzes the program's data, launches the uninstaller and shows you a list of leftovers (files, folders, registry entries related to the application). The application does a pretty good job at identifying them, but you should still check the results before selecting everything. "Even if you have a broken installation, Revo Uninstaller scans for an application's data on your hard disk drives and in the Windows registry and shows all found files, folders and registry items so you can delete them."
Revo Uninstaller also includes a junk files cleaner and a way to remove the history from browsers, Microsoft Office and some Windows application, but CCleaner is a better application for this.