Remote admin of a win2003 server

We have a server that contains all company software and data. For technical reasons, we are going to use a consultant to provide external support to the company and he will need access to the server across the Internet.

Internet access is provided through an ADSL router with a so called Firewall but it’s very basic.

What is the best method of providing remote access securing specific devices by user ID and password without adding other hardware or software?

Thanks in advance for any suggestions.

Kind regards.