Upgrade help from 1.5.x to 1.6.x

i turned off all the core plugins or so i thought, i disabled blogs and all that or did i miss something? once i turned off the plugins, i replaced all teh files and did the upgrade.php. everything went fine except i can't login anymore, said an error message something about not activated. i'm upgrading using xampp via linux to test it out on my computer before going live with the upgrade. so what am i doing wrong?

i'm thinking about upgrading using my web server instead of locally but on another part of my site.