hi
Assuming you are working with a live server that you REMOTE DESKTOP INTO and not a shared hosting account here is what you will need to do:
Let's assume your file system looks like this:
c:\inetpub\wwwroot\www.foo.es { some html files in here}
c:\inetpub\wwwroot\www.foo.es\bar {some html files in here}
Without any custom ISAPI filters installed, you would do this by setting up two WEBSITE's in IIS.
The first website would have a home directory of c:\inetpub\wwwroot\www.foo.es and the 2 host headers "www.foo.es" and "foo.es"
The second website would have a home directory of c:\inetpub\wwwroot\www.foo.es\bar and 1 host header "bar.foo.es"
Once you DNS has a CNAME of "bar" that points to your server and you clear your local dns caches, this should work. If you don't know how to make the DNS change the easiest thing you can do is log into to your domain name control panel from whoever you purchased the domain name with and do the following:
ADD A CNAME RECORD: For the host enter "bar" for the points to enter "foo.es" and you should be all set