copyright law can be a little bit different depoending on the country you come from, or the country or origin that contract was drawn up in so you need to check in your legal domicile, but I think the UK & US are largely the same.
In the UK defacto you own the copyright of anything you have developed or written unless:-
you have written the software as part of a contract for a third party where the contract expressly refers to ownership of the copyright. ie your customer may insist as part of the contract that they own copyright of work completed by you.
you are employed full or part time by the company (ie you cannot claim copyright on code if you created the work, or part created the work on a third parties time and or using a third parties equipment. Ie you cannot write some whizzy bit of code at work and then seek to claim copyright on it, nor can you use a company computer at home for private development.
you explictly assign or disown the copyright... ie you make an express assignment of the copyright to a third party, or you make it avaible under things like the GPL.
you cant 'grab' copyright of someone elses work by incorporating it into your work, and neither can your customer. so you cannot claim copyright of code written by someone else and incorporateed in your code even if you modify it slightly or significantly.
some of these limits are hard to police in the software world...... how do you proove where a piece of work was done for example, how do you prove a particular function or routine was written on your time or on someoneelses time, or even how do you prove that you wrote to similar functions doing the same thing but for different appplciations, after all if you wrote them contemporaneously then defaxt they are going t be similar (after all its the same person solving the same problem)
you dont need to actively assert ownership of copyright, its granted de-facto, however its no bad thing, it runs for (I think) death + 50 years. The assertion is best using the approved form eg "Copyright 2007 Joe Blogs Inc: all rights reserved"
the costs of protectiong your copyright are going tobe horrendous. if you are a one man band going against virtually any corporation or limited company you are going to loose regardless of the rights or wrongs of the matter.. they will almost cetainly have the cash to outspend you on the legal parasites, and cash does buy judgements in this area (a bit like libel). So take prudent steps elsewhere to protect your IP. make sure you never deploy anything except as an MDE, consider wrapping key or whizzy functions into VB DLL's. dont give design rights to the MDB to the end user. if they want to generate their own cusotme reports or forms then fine.. but let them do that in another Access model.
if I incorporate code on customer sites, that the cusotmer can modify or read, that is mine I usally include a message in each relevent section asserting my claim to copyright and an express permission to use or modify that code. if for no other reason then the phone may ring in a few yers wanting more work doing.
I'd rather be riding on the Tiger 800 or the Norton