You can use a software called Pen Drive Linux. The software can be found at: http://www.ubuntu.com/download/help/create-a-usb-stick-on-windows .
Once you setup your usb drive to be bootable with Ubuntu, you can then reboot your system (catch the boot menu, and boot from usb). It will let you run a liveCD version of ubuntu which is fully usable. You can use the OS to your hearts content. You can then install from the liveCD or just shutdown, remove usb drive, and start your computer like normal.
FYI people use liveCD's to do banking in a clean environment all the time. So when you make your usb drive, even if you don't use it keep it. You can always use it as a clean enviornment for shopping or online banking.