Ubuntu makes it very easy to create a USB drive if you already have it on your computer (note: This was implemented in version 9.04, it may be in the repositories for versions before that). All you need to do is click "System", then "Administration", and then "Startup Disk Creator".
Once you have completed the process, your Ubuntu should now be on the USB drive and you can boot from it. However Ubuntu is quite a big OS, requires a 2GB USB drive, and takes quite some time to boot on older hardware. If you are after a fast boot, I would not advise using Ubuntu; instead use a smaller distribution that uses the computer's memory like the ones I am about to show you below.
However if you need to do some media editing or something similar, Ubuntu is definitely the distribution that you will need.