How to get a list of all users in SharePoint

All users that exist in a SharePoint web (but don't necessarily have permission):

SPWeb.AllUsers

Users that exist in a SharePoint web but have been granted some permission:

SPWeb.Users

Users that exist in a SharePoint site collection (but don't necessarily have permission):

SPWeb.SiteUsers

Note: I am assuming you are using WSS 3.0 / MOSS 2007 but this should work for WSS 2.0.

Also look at:

SPWeb.AllUsers vs. SPWeb.Users

SPWeb.AllUsers vs. SPWeb.Users vs. SPWeb.Groups

eg:-

Dim Site As New SPSite("SiteURL")
Dim AllUsers As SPUserCollection = Site.RootWeb.AllUsers
Dim u As SPUser
For Each u In AllUsers
    Response.Write(u.LoginName & " " & u.Name & " " & u.Email & "<br />")
Next