How to automatically redirect affiliate link to a certain page

Hi,

I'm working on an application that has an affiliate program. Say John Smith signed up as an affiliate, and he gets a link:

http://www.mysite.com/johnsmith

Now, when people click on this link, I need ti to automatically redirect to: http://www.mysite.com/Signup.aspx

How do I set this up? In IIS? Or in the application code?

If some one can provide some insight/ help/sample code, that'll be very much appreciated!

Thanks,

Claudia