phranque - 11:12 pm on Apr 7, 2012 (gmt 0)

you might be ok if you move the url structure to the root directory of a subdomain and then it would be a simple matter to host the forum subdomain separately from the main hostname.
http://forum.example.com/

otherwise you could use mod_proxy or similar to send all requests for http://www.example.com/forum/ to the ASP server.

or use mod_aspdotnet:
http://mod-aspdotnet.sourceforge.net/ [mod-aspdotnet.sourceforge.net]
or mod_mono:
http://www.mono-project.com/Mod_mono [mono-project.com]