InfoPath Dev

1)Someone makes a new infopath file and puts it into sharepoint.
2)Now, i need to open it, change something and click a button, which will send this version to sharepoint AND will save it to harddrive.
I'm wondering how to save a file from inside this file?

You should be able to use the SaveAs() function inside a custom submit procedure, but saving to the local hard drive will require full trust.

I had a problem with XmlDocument = XDocument. But i have done it like this:
XmlDocument o=new XmlDocument();
o.LoadXml(thisXDocument.DOM.xml);
o.Save(@"c:\xml\testXML.xml");