Follow these steps exactly after burning the ISO to DVD using ImgBurn at 4x speed:
Reinstalling Windows 7
You should have used Partition Wizard bootable disk which will not fail, while the installed version and any other Disk Manager besides Disk Management will fail.
Read and follow all steps carefully, ask back if you have any questions because a simple mistake like missing that your are to use the PW disk can ruin your system. There is no substitute for thoroughness in these operations, there are no shortcuts, and no steps can be ignored or missed.