User Registration

I know this question in general has been asked before and various answers have been given, however is my case, none of the answers I found really apply.
This is the scenario - I have written a very small commercial program. This product will be available for payment and download electronically.
I generate a registration code at the time the program is paid for. And that registration code is stored in an access database on my web (external ISP) which is in a directory with read write modify permissions having been set. At the time the registration code is supplied, I log the IP address the user came from.
Now my question is this - when the user sets up the program (an asp application) the registration code is entered and stored in the users database. Now when the code is accessed, I would like the code on the users machine to access my database, and check the user registration details are the same - if they are not the same then the program becomes limited in its use. So what I am asking is how do I programatically allow external access to my database from another machine to achieve this ?