Virtual Directories

[Yahweh]

I'm creating a website where registered users have their own folder to store files. I plan to create the directories like this:

/root    /images    /includes    /users        /bob        /joe        /jack        /yahweh

This way, all the files are stored at www.site.com/users/username/.However, those kinds of urls are tacky and ugly, I want users to be able to type www.site.com/username/ and get to the same place. I can't use username.site.com, because my host is cheap and doesn't make ASP.Net available for subdomains.I think virtual directories can be simulated with 404 pages, but I'm not sure how to get started. Is there a painless way to forward all www.site.com/username hits to their corresponding www.site.com/user/username directory?