Hi, I had the same problem when I tried login in my moodle's installation: in some computers I could login and others not, so the problem was not about the installation.
The question is that when I'm under a firewall I can't log, in the same way that I can´t post in a forum or create a new course, for example. If I disable the firewall then I can log and do anything without a problem.
I think that's the problem (although it sounds a bit strange) or at least for me it was.
Cheers