[2.0] Connecting to a server securly

I'm developing a piece of software that won't be used in a controlled environment (i.e. it won't be on a work network or anything, just on anyone's PC) and I'm having trouble trying to figure out exactly how to implement a server connection.

Basically, a user will need to login when they first open the application. This means the application needs to get data from the server (the data would be stored in a database). The application will also be able to create users (which, of course, needs to be stored on the server).

What's the best way to accomplish this? The server I would prefer to use this with is a Linux server and I don't have access to install anything so I was thinking maybe some simple PHP scripts to pass data to and from but the problem with this is that it isn't exactly secure (anyone could figure out the pages and send data to them to create users). I could write my own application to handle the server stuff and put it on a Windows box or something but rolling my own requires extensive testing and one day I would like to be able to process transactions through this.

Wow! A five-star thread, even before it's got any replies.

I feel like I'm the only one who uses them, lol