I just had this MSDN paper/article pointed out to me about Auto complete in IE. It goes into mentioning advantages, implementing and security of using auto complete, along with how to prevent it from happening. Two examples they give are

<INPUT TYPE = "password" NAME = "password" AUTOCOMPLETE = "off"></br>

to disable a field, and:

<FORM AUTOCOMPLETE = "off">

to disable auto complete on a form.