Restart the computer and enter the BIOS/Boot Menu (usually F2, F10, Esc, or Del) to set the USB/DVD as the primary boot device. When prompted, press any key to boot from the media. Select your language and keyboard layout. Click "Repair your computer" (do not click "Install Now").
Choose "System Restore" from the System Recovery Options to restore Windows to a point before the error occurred. windows 7 bootmgr download