在IIS7 404重定向页面中访问原始URL

I have an .aspx page as my custom 404 page on a site set up on IIS 7. I need to retrieve the original URL that the user was trying to access in order to do some processing on the 404 page. The trick is that I need to specifically handle 404's that do not contain a .aspx extension (e.g I have an .aspx page as my custom 404 page on a