for a Major German bank a few years ago. It was based on a server in Singapore, and the client wanted individuals of high net worth to be able to remotely view their portfolios in a secure personal area. Security was absolutely paramount, and my work was scrutinised by a couple of web experts from one of the big accountancy firms before it could go live.
I did it by creating a separate sub-web for each client; essentially a mini web site within the main site. The client areas contained information that the bank thought would be specifically of interest to them, and they were able to access pdf files showing the state of their transaction accounts as at the close of business each day.
It was all controlled by custom-written scripts that controlled the way each person entered his of her area. The individual could only enter from a specific page on the main site that was available only to clients,and only by entering a username and password. If anyone tried to access the areas from another location the site locked them out.
I can't go into detail for contractual reasons, but it was an interesting project, to say the least.
If you really want to do something similar you're in for a massive learning curve - it's a complicated procedure, and you have to get it right, or - as fourm member has said - you'll run foul of the Data protection legislation. You will also have to register with the Information Commissioner as a data controller.