Can you recheck your figures please. I think you are mixing up your MB, Mb and GB.
For W7 you need 16 GB available hard disk space (32-bit) or 20 GB (64-bit) plus room for other programs and all your personal Documents. In addition a small System partition (approx 100MB) is created at the beginning of drive for the Boot Manager
What makes you think that the faulty version was left on the Hard Disk?
how big is your harddrive? 250GB, 500GB or what?