Train your computer to understand your voice using Windows Vista
With Windows Speech Recognition, you will be able to use your voice to
perform common tasks, and enjoy the convenience and accessibility of
dictating commands, e-mail messages, and more.

Windows Vista is no different, but one new feature has proven particularly
popular: speech recognition.

Windows Speech Recognition is a valuable usability tool with which you can
literally interact with your computer through speech—an especially helpful
feature for people who have difficulties operating keyboards or mouse
devices. Instead of relying on the standard keyboard and mouse to manually
manipulate the computer, you can train Windows Vista to recognize your
voice for interaction. You can compose a letter or e-mail message by
dictating its content to many commonly used applications. In fact, you can
perform most common tasks on your computer by voice.

read more please visit here
http://www.microsoft.com/windowsvista/community/speechrecognition.mspx