In Internet Explorer, the "register/submit" button in the registration form webpart is not firing an event in IE, it works fine in both Firefox and Chrome.
Do you have a conflicting version of jQuery you are using for your site. The registration webpart includes a jQuery library and plugin if I remember right and if you have another version, it could be causing problems.
I believe it's the \CMSScripts\membership.js file. You might check the dev tools in the browser you're working with and see what the JS errors are as well.
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