Ah, you don't mean the page cache, then, but rather the AutoComplete feature of IE.
First, it's really weird that it has a problem with autocompleted data. My guess is that it must somehow not be triggering some Javascript on the client that needs to be run (though I know almost nothing about ASP.NET -- Forum855, btw).
The only way I know to overcome AutoComplete is to not provide a form field name that the browser recognizes. For example, if your zip code form field name is "zip" then anything the user has ever entered in a form field called "zip" will be available via autocomplete. If, however, the form field was called "zip8378378490" and you used a different random number (or incremented number) every time, there'd be nothing to grab autocomplete info from. Also in the form hide a form field that contains the number appended to field names on the page, so that you'll know what to refer to on the server after submission.
Which is a fair bit of work, certainly, if your problem is a bit of Javascript. The paragraph above is a nifty solution (imo, tooting my own horn) for more dire AutoComplete problems (like forms where users enter sensitive information), but might be a bit much for your little problem. I suggest asking the question over on the ASP.NET forum first.