well if i were you, just get a Windows 7 64 bit copy from the store, and reformat your drive. to answer your other question, and correct me if i am wrong (experts) but i believe if you "Installed from microsoft" the whole process, formatting and all would be done. because when you install a new os, unless the drive is partitioned and split in half all old data is gone from that hard drive and a new bootable format is installed.
I am going to school for hardware a+ certification and Computer Programming so no i am not a expert on these sorts of things. but you catch me in the graphics forum, i can handle any question asked.