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It shows how in Mark Partition Active Method 2 to use the Command Line from System Recovery Options - Try Option 1 from the F8 options Command Line first.
If that won't work and you don't have a Win7 installer DVD or bootable flash stick, or System Repair Disk then you'll need to download the installer ISO from Step 1 and burn to DVD or write to stick from steps 2 of Clean Reinstall - Factory OEM Windows 7. Boot installer to access Repair My Computer on second screen.