Just a couple thoughts from a non-expert. First, find the Linux distro and desktop that is most to your liking. This takes internet searching and experimenting.
Second, you have an Athlon 2 with 4 Gb ram. I'd go with a 64 bit distro (Linux Distribution). Most often, this would be better.
Third, partitioning a new hdd. Most people DON'T choose the default install. We create a boot, root and home partitions at least. You really have to read about this and let it sink in.
Forth, Windows. I used Windows since 3.1. It does a typical install that is great for the masses with little concern for security and individual needs. With Linux there are so many choices and so much to learn right from the start, that people do get overwhelmed. Stick with it. 6 months from now you will thank me for the encouragement.
Fifth, USB drive? I don't even have a USB drive. Just my trusty ol dvd burner. I download my Linux distro from a known site, I compare the md5sum with the builtin Linux utility to verify there are no bits out of place during the download process. I burn that .iso file to dvd. k3b works so nice. It verifies the md5sum upon completion. From there, I plug that dvd into the drive and reboot to that live version. Check it out and install if I want to. USB must be basically similar in procedure?
mmmmmm Olive oyl.
i5 3570k, ASRock z77 Extreme 4, 8 Gb Ripjaws 1600, Antec 430w psu, HVR 1600 tv tuner, custom case- marble top, oak face. Carver & DCM Time Window sound system. Mint 14 KDE.