I would do a fresh install instead of upgrading over the accumulated debris of an OS that does things in a different way.
You can create the USB media either for your existing computer or "for another computer", mostly it's about drivers and maybe it will transfer the license as well. I use the generic one all the time. Get a new HDD just because and install using the Win7 license code on the sticker, taking care you choose the right version. Home to Home, Pro or Ultimate to Pro etc.
It should take about an hour or so depending on the speed of your computer. After that you'll have to install the motherboard drivers, then the external card drivers (video). It's pretty straightforward.
Oh, and: You will be strongly suggested to use a Microsoft account, the option for a local account is in the lower left corner of the screen on a low contrast small font. When the installation is finished, you'll be asked to use speech recognition and to allow all kinds of ways to spy on you for more effective marketing. Simply click the chapter saying "No, I don't want to..." and click Next. There's seven such questions before you get to the Desktop for the first time. There's more privacy related settings in the Settings, but disabling those is a good start.
