Please realize that the answer selected will erase all data on the HD.. Hope you dont have anything important on it.
My reply can be shortened like this: "No, you don't have to wipe the drive, the installer will do it for you."
What I didn't add, but also worth mentioning, would sound like this: "No, you don't have to wipe the drive, if you don't want to. If you do an upgrade instead of a clean install, your existing files will be moved into a folder named Windows.old."
But if you have important files on the disk, I can only repeat what lafontma wrote. Back them up on another medium, and after that, you can do it either way.