If your subhost is on you box and you configure IIS for it under Advanced button in the IP section of the general tab, then the hits will be going to the default page in the folder you specified under the directory tab.
From your post, it sounds like your fourm is being hosted else where in which case the IP address in your DNS "A" record should be pointing to the ISP's IP address. Now if the ISP is using host records and vertual resolution then they too will have to configure their server with your host.domainname.com before the hist will make it in.
Keep in mind just because Win2003 comes with DNS service does not mean the root domains are pointing to your dns server for name resolution. It is possible that your upstream provider is serving DNS to your box and your is never being hit. You should be able to check this by doing a traceRt or looking at the log files.
--hm