Since no one else has mentioned it--------try clearing out your browsing history, cache and cookies in IE. Then re-start IE. See if that has any effect on the problem. If not, try using a different browser like Firefox or Chrome and see if you can duplicate the problem. Corrupted, conflicting or expired cookies can cause a lot of weird problems.
Are you using any add-ons or plug-ins in IE? Maybe a Theme? You might try disabling them one at a time, then try signing in to your email again after each one. That would at least eliminate any of them as the source of the trouble. Extensions to your browser are not usually vetted by the browser developers. So third party add-ons etc can cause unexpected problems too.
