This question is a bit ambiguous. When you say access an internal share from an external machine, do you mean (A) access a share on a private network share from the internet? Or are you asking how to (B) access a share on a domain from a machine that is either not a member of that domain or is a member of another domain?
If it is A, is the machine behind a firewall on a private network or does it have a public IP (and then is it ?live? on the net or behind a firewall)? No matter how it isconfigured, if A is the scenario, I would recommend ensuring it is behind a firewall and setting up VPN to access it. Having direct access to a box with network file sharing or opening the ports to allow this would basically be like posting a message at alt.2600 saying ?Please come destroy everything on my network.? Another option, if you just need to make some files available from the internet would be to setup an FTP server. Windows has the ability to do this, though it is flaky. If you wanted to go this route, I would recommend a third party FTP program (Linux would be better, but doesn?t sound practical for what you are trying to accomplish). Stick it behind a firewall and use NAT to reroute traffic coming in on Ports 20 and 21 towhichever box is functioning as the FTP server. Bear in mind that this poses a sizable security risk, so you should setup the FTP server to use encryption
If B is the case, are you on the same physical network? The same subnet? The first answer to your question gives some information if this is the case, so without more specifics, I will not go into more detail on this.