Well the problem is that either you have wiped Windows 7 off or upgraded Windows 7. The only way to go back to Windows 7 is to reinstall Windows 7. If your laptop/desktop had a windows 7 recovery disc use that. If it didn't came with disc maybe try contacting your PC manufacter but I think it is too late. If the manufacter don't give you the disc then you would have to buy Windows 7 disc I afraid.
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