If you are trying to downgrade from Windows 8 to Windows 7, you may run into the issue above. Here is how to fix it.
Boot computer to the Windows 7 installation disk.
At the main screen type "Shift" "F10" (my may need to type "fn" "shift" "F10" with some computers"
A command prompt will open
type diskpart and hit enter
type list disk and hit enter (it should tell you the disks on your system and their number. Default is 0)
type select disk 0 (or the number you found in the above step. 0 is just the default) and hit enter
type clean and hit enter
You should now be able to install Windows 7 without any problems!
Please comment if this helped you. Thanks!
-Master Fink