I agree with others that suggested Linux Mint - it is probably the most complete desktop distribution. If you don't have more than 4GB of RAM, and don't intend to add RAM, you can download the 32bit version. Linux Mint offers a choice of desktops: Mate, Cinnamon, and XFCE. Linux Mint 16 XFCE is currently a "release candidate" or RC so you may want to wait until it's released. XFCE is the most lightweight of all the above desktops, but has some minor drawbacks in the usability department. If you have 1GB or more of RAM, Mate and Cinnamon are good options.
A utility named Unetbootin is available for Windows (and Linux). You can download and use it to prepare a bootable Linux USB stick. Plug in a USB stick, run Unetbootin and point to the ISO image you downloaded. The program does the rest (be patient, it takes a few minutes). This way you can download and test different distributions, or for example Linux Mint's different desktop versions (Mate, Cinnamon, XFCE).
Don't forget to download and look at the Linux Mint manual, it can be very helpful for beginners. Good luck!