If you are an employee you probably have something in your employment agreement that states that the company owns the code you write on their time and/or their equipment. That has been in every employment agreement I've signed. Usually independent contractors have the same thing in the contracts they sign with companies and, as Matt has already said, you need to have exclusions put in for generic code you bring with you. There is actually a fairly large placement firm in the US that I won't sign an agreement with because of the language about code ownership and I don't think anyone should because it basically says that the company you are placed with will own all code you write or use while on assignment there, which means you can't use Glenn Berry's performance scripts or Brent Ozar Unlimited's Blitz scripts.