I did like you said... My Computer> Tools> Folder Options> View> Advanced Settings> Use simple file sharing (Recommended). Then I went to, not only the file I'm trying to share, but I experimented with five different files and none of the Sharing properties give me a different dialog on the Sharing tab. I have XP Pro 2002 Service Pack 1, if that makes a difference.
As far as I can tell I'm the administrator, but I'm wondering if there's any way I can be sure of that. Everyone who uses windows on this computer has a profile but there are no login passwords.
I've actually seen someone do what you're describing before and it worked for them, but every time I try it myself it doesn't. There's gotta be something I'm doing wrong.