Hi there,
To find the Command Prompt (cmd.exe) first click the Start button. Then at the start menu, click All Programs, Accessories, and Command Prompt should be listed under that folder. Remember to right-click it and select Run as administrator if you are using Windows Vista.
An alternative method is to type in cmd.exe, or command prompt in the Start Search box.
Edit: You might want to try the Repair option in the Control Panel as suggested by Tech first, it fixes the problem most of the time.