How to connect & upload files to a free web server using C++?

Hi, I am currently working on a project thats main aim is to develop a software application that allows users to secure their data on their home computer. However, after working on this project for a month now, I have conducted a feasibility study and I realised how restricted this solution would become. Therefore, I have decided to change the project solution to be more flexible and available for users.

The idea is the users will run the application and be expected to authenticate themselves to a server. Once authenticated they will then have the functionality of uploading/downloading their important files to & from the server which would then allow them to access these files from any desktop running my application.

I am basically looking for any type of solution that prevents me from having to write my own webserver. Just an application that allows users to upload/download files to/from a web server via authentication.

Therefore, I was wondering if it would be possible to create an application that can connect to already available free file servers on the internet and provide this functionality through my application i.e. users cannot connect through internet browser?

Or could you suggest any other alternative solutions for my idea?

I have had a quick look at CardSpace, was wondering if I could encorporate this type of technology into my application aswell?

Thanks,
James.