You can run the Ubuntu OS off a memory stick. If you use Windows, you should do that, that's what I described initially. Just download the OS to a memory stick, put it in your USB port and when the machine loads up hit F-whatever it asks to get to setup or bios and change the setting so that it loads off the memory stick instead of the hard drive.
Are you able to switch back and forth to Windows easily? Like, would you just have to restart your computer in Ubuntu and then just hit F8 once it's starting back up?
I don't think you even need to do that. If you boot from the memory stick, all you'd have to do is shut down the computer and remove the memory stick and then start back up... should default to Windows.
The way this works is based on how the boot order is defined in BIOS. So... you can set the BIOS to attempt to boot from an external device (USB stick) if a bootable external device is present to boot from. If no device is present, it will move on to the next option (e.g. Windows).
I believe Windows factory installs probably don't attempt to boot from external device by default, hence the need to press F8 or whatever when BIOS comes up on startup and manually select the boot drive.
So... when you DO boot from the USB that you have installed Linux on and configured to be bootable, you are literally just running from that drive... your hard disk that is partitioned to Windows is still partitioned to Windows. You can INSTALL Linux from a USB as well and overwrite some or all of the Windows partition, but you'll definitely know when you are doing this... not gonna happen by accident.
What k. is describing just keeps anything you do from the USB booted OS local to the USB drive itself. I mean, the data traffic still goes through your PC network card blah blah blah,
but data accessed and saved will be written to the USB drive.At least that is my understanding of it.
Edit: I am actually not sure about the portion struck out. My experience is more booting from a USB to actually do an install, so I am going to shut my yap and let somebody who regularly does this explain if need be.