Login problems

Hi,
Noticed I have a problem logging in with IE.
The problem, as far as I can tell, has something to do with my
domain masking, as it works ok when I type in the actual url,
rather than the domain name. Took me a while to  notice, as I
generally use Firefox, which doesn't produce the same problem.
When I attempt to login, it redirects me to the index page, where it
says 'not logged in'.
When the correct username and password are entered, does it simply
refresh the current page, or does it load index.php ?
Any advice would be appreciated.
Thanks.