Webbased database - maximum protected login

We need a webbased database system that will allow users to log on to securely. Every user has access to his client's files. Users must be able to modify certain client data fields. Clients must be able to add data to their file. A program must be build that will collect data from several existing applications' database on a client's Windows PC and send it securely over the net to the server. The whole system will be developed to allow the possibility to easily add other applications (on the client's PC) to it.