If you're using C# with file.copy you have to have the remote drive mapped to your computer and you have to be authenticated. I'm betting the problem you are having is simply related to the fact that you don't have permission to get the file you are trying to get.
If you have a valid username/pw for the remote machine, map the drive:
http://windows.microsoft.com/en-us/windows/create-shortcut-map-network-drive#1TC=windows-7
If you still have issues it's likely something on the remote machine stopping you.
Edit:
If you are attempting to access a drive on the same network attempt to visit the URI in windows explorer. If you can get to it than there is another issue entirely. If you cannot get to it or if it asks for user or PW you simply don't have permission to be there.
Edit 2:
Pro-tip: Avoid trying to look like you are attempting to "hack" something. I guarantee you you will be flamed and down-voted to oblivion as obvious by the other responses you've gotten.
EDIT 3:
Another stack overflow article talking about the same thing with c#
Accessing a Shared File (UNC) From a Remote, Non-Trusted Domain With Credentials
