- Open and run CMD as Administrator.
- Type Diskpart, press Enter.
- Type List Disk, press Enter.
- Type Select Disk # (where # is the number your USB drive shows up as), press Enter.
- Type Clean, press Enter.
- Type Create Partition Primary, press Enter.
- Type Active, press Enter.
- Type Format Quick FS=FAT32, press Enter.
- Type Assign, press Enter.
- Type Exit, press Enter.
- Copy everything from the Windows 7 installation DVD onto the USB key (a simple drag and drop will do; if you have an .iso extract or mount first).
Make sure put USB HDD as the first priority of start up.