In Internet Explorer there is a feature call AutoComplete. It remembers what you type in forms on web pages. I suggest you delete the information it has saved, and it will then remember the correct information next time you fill in a form. Go to Tools -> Internet Options -> General -> Browsing History - Delete -> Delete forms... (these instructions are for Internet Explorer 7).
On the other hand, you might be using an add-on toolbar.
On the Google toolbar (version 4.0.917) there is a feature called AutoFill. To change your address data click on the Settings button, Options..., then AutoFill Options...
On the RoboForm toolbar (version 6.7.9) there is a feature called Fill Forms. Click on the RoboForm button, Tools, Edit identities, then Address.