I wrote a quick script to pull down a live drupal database on to a machine on a local network, and that works fine.
I also made a copy of the actual Drupal instance on my own machine.
I edited settings.php to use the db on the network, and everything appears to be working correctly when I run the site from localhost, except I can't log in.
At first I thought I should just change the password directly using MySQL, but that didn't work. Then I tried running update.php, which also didn't work.
I'm stuck, why won't it accept the username/password? I have double checked that I'm using the correct login credentials on the live site, and it works fine.
Edit: The site was duplicated from a Linux server onto a Windows 7 machine, if that makes any difference.