An MS-DOS startup disc is not appropriate for Windows 7. You need to use a Windows 7 System Repair Disc which can be created with the Backup and Restore utility when Windows is running okay.
Your immediate problem, however, is that you cannot get your PC to boot from the CD drive.
You may have to go into BIOS Setup and set the CD drive as the first boot device.
Or invoke the Boot Device menu, select the CD drive and press <Enter>