about apache asking client certificate

I am using apache as my web servier with ssl enabled. What happened is
that when the user uses IE to open the first page of the site, it
requests the client certificate by poping up window to let user choose
one certificate from the list (The list in the winodw is actually an
empty list). However, if using firfox, there is no such a problem.

Also if the user put url in the IE browser address line again and
refesh the window, there is no such a window popup to ask client
certificate.

Why it behave like this? I want to get rid of the popup window that
asks client certificate.

Can anyone give me a help?

Thanks,

David