It is highly unlikely your motherboard will support booting from a USB device.
You would be better off buying a USB hard drive case and getting your own internal drive, you will have an option to pick the size and you can swap drives when you want to.
Newer to computers:
Just out of curiosity...I'm thinking about extra storage and flexibility in the case of something ever going wrong with my computer. I've looked at internal and external hard drives. If I were to buy an external (usb), such as an Iomega 40gig, would it be possible to install Windows XP on it and use it as the main hard drive. New to all this stuff. Thanks for the help.

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