Help with virus/worm!!! How do i reformat?

My anti virus didn't work. I tried avist and I am now going to get another anti virus but I have been told that I should reformat my whole computer and start from scratch! Please someone help me out here and tell me what to do.!!! I don't know how to reformat????
Update: It is Windows XP Do I reformat before I down load the anti virus protection?
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