your username is your account name.... the name you use to log in with.
Also, if the drive is already formatted, which your drive IS, vista will NOT format it, you will need to tell it to format if you would want it to format. You don't want it to do that, so if you simply follow the normal install, you will not format your drive.
I had to reinstall my vista two weeks back, onto a different drive. My old drive, which had vista still on it was still attached though, and vista install noticed that it ws actually there, and saw that it had an install of vista on it, and therefor created a folder on the new drive with windows.old on it. It had all my old stuff in it. (though that was also still on the old drive). Anyways, what I'm trying to say is that vista WILL NOT format your drive without you telling it to explicitly.