For when you are unable to log on to the computer, the recovery disc is able to run Startup Repair, System Restore and other options such as run the command prompt. This disc will work for Windows XP, Vista and 7.
To use it, download the iso file (see below), burn it to disc and boot from it.
Note: You can not install Windows with this disc. You can only use the recovery options.
Download
Windows 7 Recovery Disc (32bit)
Windows 7 Recovery Disc (64bit)