Virtual directory creation:

I have a website under IIS that has guestbook fetaure and other stuffs..
and from one of the ASP pages I am allowing the members to create their own guestbook....and I want the URL for the GuestBook to look like http://membername.siteurl.com/gbookname..
so that every time someone creates a Gbook they have their unique...url...but i am getting error when i am trying to create a virtual directory(from a asp page) that will just point to the already existing gbook files..

any help???