What is the best anti-virus program?

I have a Dell, Windows XP, and I know next to nothing about computers, aside from how to use programs on them. Don't give me technical answers or anything or I will be LOST.

I just know I need a new anti-virus program, and I'm wondering which I should buy.
Update: By the way, I have no problem going out and paying for a program. I just want one that works, and if it's not free, it's still cheaper than having to buy a new computer because it has a bunch of problems on it.
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