Can I use my PC to install linux/ubuntu to my external drive to be used on another PC?

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http://askubuntu.com – Long story short. I have a decent PC in the other room that has no internet. So I want to install Linux/ubuntu onto a external 1.5TB hard drive so I can run the other PC on it. How ever, I do not want linux/ubunto to effect this PC at all. Please help. PS: in case you cant tell, I have no idea what Im doing. I am computer friendly however I am very unfamiliar with Linux/ubuntu. Im not even sure I know what it is. I just cant afford to buy windows 7. (HowTos)