Simulate directory for each affiliate and then redirect

Sorry for the ambiguous thread title, I'm not exactly sure how to describe what I'm doing. Let me try...

We're starting a customer affiliate program that allows our customers to earn merchandise credit by sending their friends and family to our website via a special link. The link looks something like this: http://www.c28.com/?adid=st&iid=6950.

Not exactly the easiest url to remember, right? I think the program would be much more successful if each customer had their own directory that redirected to the ugly url. Something like this: http://www.c28.com/username.

Obviously I can't manually create a directory for every customer and a redirect. How would I automate this? I know it's possible, I just haven't been able to find any answers.

We're using regular ASP and IIS 5.0. Any help would be much appreciated