In the company that I work there are computers that are connected to the Internet and computers that are connected to a Local Area Network. The LAN is considered a "safe zone" and the files that reside there should never be copied/moved to a computer that has Internet Access. So, now, if we want to download an installer for an application for example, we download it in a pc that has Internet Access and then move it using a "secure USB stick" to the Local Area Network.

Is there a way to create an "safe, one-way connection" between a computer with Internet access and a computer from the LAN? This practically means that only files from the computer with the Internet access can be copied/moved to the LAN. In addition to that, if you want to transfer files you would have to provide your security credentials for the network (so, that only users with the appropriate access levels will be able to transfer files).

Is it possible to create something like that and make it completely safe (or at least "equally safe" with the USB method that we currently use) or the fact that the computer with Internet access is connected with a wire to the LAN is a security risk by itself?

NOTE: the LAN setup involves 2 Windows 2003 servers with Active Directory, Web servers and pretty much all the services that you would expect to find in a Windows network.