IE7 caching is killing me!

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Before the IE7 update, I had no problems with my site structure. A
member would log in (which would set a cookie), and then the upper
corner that says "Welcome Guest" by default would change to "Welcome
(whatever the member's name is), you have X Private Messages."

After the IE7 update, though, it appears that the "Welcome Guest" part
is staying in the user's cache. Technically, they are still logged in,
but they have no idea that Private Messages are waiting for them.

Any suggestions on how to prevent this? I currently use these meta
tags to prevent caching, which worked in 6.x, but not now:

(These are in the header)
<meta http-equiv="cache-control" content="no-store, no-cache, must-
revalidate, Post-Check=0, Pre-Check=0">
<meta http-equiv="expires" content="0">
<meta http-equiv="pragma" content="no-cache">

(This is between </body> and </html>)
<head><meta http-equiv="Pragma" content="no-cache"></head>

The only solution I can think of is to run the entire site through a
security certificate, but it already has the occasional speed issue so
I really don't want to do that unless I have to!

This is the site:



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