I had this problem a couple of months ago. Here is Jason's explanation/fix for the problem. It has worked ever since...
Hi Dave, There is a bit of a problem here in GForum - when the User table gets too large, there is too much on the page for a "GET" request, and so IE just refuses to do anything when you hit the submit button. I changed over the relevant admin template on your site to be a POST request instead (which has no size limit), and tested it out; it's working now.
As for that other page, you can try changing the <form method="GET" ...> to <form method="POST" ...>. If it's the same error that'll fix it ("GET" is the default). -- Jason Rhinelander -- Gossamer Threads, Inc. Hope this helps. Dave