I had the same problem (“cant’login”) in my local pc with Windows XP sp2. I installed WordPress 1.5.1.3 on XAMPP Lite version 1.4.14.
It was a cookie problem. I solved it only turning off the “Ad Blocking” and “Cookie control” of the Zone Alarm Suite that I installed on my machine.
I don’t know if this can help.
Bye,
Cristian
