Many newer computers can start a computer from a Live USB Flash drive which has Ubuntu 12.04 preloaded on it. All you do is enter your BIOS/CMOS in your computer, and tell your computer to start from a USB flash Drive.
The Patriot Xporter XT Boost USB Drive is a great USB 2.0 flash drive for installing Ubuntu Linux 12.04 LTS on to because it is fast, durable and reliable.
You can also use a boot manager program which is like the BIOS, but it is only used for picking which device which you want to start from like a USB drive, CD, hard drive or floppy.
For Hp computers like the a1720n Pavilion desktops, you can press the ESC key during start up to launce the start up manager to tell your computer to start from the USB flash drive which you have plugged into your computer USB port.
All you do is use the arrow keys on your keyboard to high light your USB flash drive, and press enter. It typically takes a few minutesto start your computer to Ubuntu from a USB Flash drive. After Ubuntu is loaded, you can pick Try Ubuntu to try it out, or install ubuntu to install it.
Ubuntu 10.04-12.04 has a USB start up drive creator built-in to it for making USB flash drives from the Ubuntu 12.04 LTS CD ISO image file, or CD.
You can also use UnetBootin to make a USB flash drive which is bootable to Ubuntu Linux.
Ubuntu 12.04 LTS ISO can be downloaded from Ubuntu.com.