<p>Let me try to explain what I need. I have a server that is visible from the internet. What I need is to create a ASP.NET application that get the request of a web Site and send to a internal server, then it gets the response and publish the the info. For the client this should be totally transparent.</p>

<p>For different reasons I cannot redirect the port to the internal server. What I can do but no know how - maybe the answer is there - is to create a new Web Site that its host in the other server.</p>

Why won't any old proxy software work for this?  Why does it need to be an ASP.NET application?  There are TONS of tools out there (both Windows and *nix) that will get the job done quite easily.  Check
Squid
or
NetProxy
for starters.
If you need to integrate with IIS,
IISProxy
looks like it would do the trick too.