How do I repair my system?

my computer will not boot, initially i got the blue screen so i tried to fix it myself. In the process I formatted my HDD so the next time i booted the system it said missing operating system. I made a bootable usb drive with a vista system image (although i was using windows 7) but my HDD will not format to install the vista software. So now im using my sisters laptop in desperation trying to find out what to do instead of buying a new system. HELP PLEASE!!!!