I had the same issue. According to HTMLdog, the culprit is the Google Toolbar, that widget thing that you download from Google that adds a search box and some other stuff to Internet Explorer.
There is an 'AutoFill' option on the Google Toolbar (switched on by default), that will 'helpfully' highlight form fields with certain names such as 'email' or 'name'. Read about it here:
Mystical Yellow Form Fields - HTML Dog Blog - HTML Dog