You may have some application that support only Windows XP but you want to run it on your Windows 7 machine.
Windows has an option of backward compatibility for the applications that have been designed for earlier versions of Windows.
To use this feature right click on the executable (.exe) file of the application, and open Properties.
Go to Compatibility tab.
Click on Run this program in compatibility mode for: check box and select the Windows version from the drop down list. This list contains almost all the versions of windows.