Is it just this site that you are having problems with or are there others?
Am I right in saying that you can access it and view it as a guest, but when you try to sign in you have the problem described?
Could it be a firewall problem?
Mike says he has no problems with his credentials, but if you want me to try with yours then please PM me and I'll have a look. I'm using Windows 7 Professional 64-bit with IE9 Beta.
Meanwhile, have you tried Internet Explorer - Reset