Removing email address....

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I have Windows XP. When I boot up my computer I get to the user screen. It says "To begin, please click on your username" and under my username says: "426 unread mail messages" When I move my mouse to this, it brings me a pop up and tells me that 426 unread messages from 3 different email addresses. The problem is, my friends have checked their email on my system and now I'm seeing their messages as well as my own! How can I delete their accounts?