Have a look at this first tutorial to make a bootable USB stick with the Windows 7 installation media, then you may care to have a look at the additional information listed below.
USB Windows 7 Installation Key Drive - Create
To start fresh with a clean installation
or clean install with an upgrade version
of Windows 7, it may be best to over-write the Master Boot Record of any possible corruption and then use the Windows installer to create and format the partition(s) for installation, have a look at the "
option of Method One
in this first tutorial linked below.
Secure Erase / Wipe : Definition and Methods
DISKPART : At PC Startup
If you want to completely wipe, over-write the entire Hard Disk Drive of any possible malware infection and / or to get the best possible space to do the install to or just for security purposes, have a look at these tutorials.
SSD / HDD : Optimize for Windows Reinstallation
Additional Secure Erase / Wipe Information