Forward Site based on referring Site

Alright, what I am basically trying to do is forward a site based on
the referring site.

For example a domain that I cannot control is linking a pop-up to a
page on my website. I would like to be able to redirect this pop-up to
another webpage based on the referring site. But, I would like to keep
this pop-up working within my own sites without be redirected as well.

I am posting this in the VB group because the page is written in VB,
but if anyone can think of another way to do this I would be
interested.

Thanks in advance.