HELP! Problem with login using IE6 and Opera, OK in FF3

This has got me completely confused. I can login successfully from my
PC using FF 3.08, but not using IE6 or Opera 9.63. I can also login
successfully on my MAC using FF 3.08, Opera 9.64, and Safari 3.2.1. How
can that be??? I'm using the same login name and password in all cases?!

I have been able to login without any problems before. However, just
yesterday I uploaded some new and updated files; jQuery-1.3.1-min.js,
and some previously uploaded files that now use jQuery and some
scripts. But it's worth noting that the login.php page does not have
any reference to the jQuery file, so it doesn't seem likely that is the
problem.

Does anyone have any idea why I can't login using IE6 and Opera on the PC?

TIA