If it's .zip, XP and Vista should open it automatically; then find the Extract All Files button while it's open. If it's something else, like .rar, for instance, you'll probably have to download a program for it. WinAce works for most things, and you can download a free 90-day trial (which actually continues operating indefinitely after the 90 days are up... or so I hear...) from somewhere on the interwobs. If I was on an actual computer now, I'd find their site in another tab and link you to it, but I'm on a Wii.