The first suggestion would be for you to delete your browser cookies and cache. We find that this resolves 99% of login problems instantly.
In Internet Explorer, you would simply need to click on 'Tools > Internet Options' and on the window that appears, click on the 'Delete Cookies' button to delete the cookies from your computer and 'Delete Files' to clear the browser's cache. Now when you try and log in, everything should work fine.
If you're still having problems, please email us for our assistance and we will look into this further.