Hi Costi,
There's at least two common scenarios that your situation might describe.
The first is when you're attemtping to post data to a web service in SOAP
format. In this case you should look at the web service tutorials in MSDN,
create a client proxy from the WSDL that the service provides (don't worry VS
does it for you) and use that.

The other scenario is where you posting XML in a non-SOAP kind of way to
something that isn't a web service - an example might be making WEBDAV
requests. In this case you should be looking at using HttpWebRequest and the
various XML classes.

Can you describe you situation in a little more detail?

Dave