For the HP a1720n desktop, and other models of desktop and laptops made by HP, there is a boot manager which you can access by hitting the ESC key on the top left of your keyboard after you turned on your computer with your USB flash drive plug into a USB port.
I used the Ubuntu 12.04 LTS operating system to load onto my USB drive, Ubuntu has a built-in USB startup disk creator, or you can use UnetBootin which works for Windows, Linux, and Mac to make a boot USB drive to start your computer from. Both the Ubuntu USB start up drive, and UnetBootin are easy to use step by step visual based programs, so using them to make USB flash drives for starting to Ubuntu is simple. You need a USB flash drive with at least 1GB in storage capacity.
After, you launch the boot manager in your HP computer, use the arrow keys on the far bottom right of your keyboard to select your USB drive which is usually the brand of your USB drive, and not WD, Maxtor, Hitachi, Seagate, and Samsung which usually make hard drives.
Once you highlighted your USB drive, you press the arrow keys to begin the start up.
In Ubuntu, you can choose to Try Ubuntu or Install it. I recommend trying it out before installing it to see if Ubuntu is the right operating system for you.
I used Patriot Xporter XT Boost USB Drive for this tutorial.
I used the Patriot Xporter XT Boost because it is one of the fastest USB 2.0 flash drive, so it will start Ubuntu faster, and make it run faster compared to using a older and slower USB 1.1 to 2.0 drive.