Try logging in one more time. If it still does not work, try resetting your password once more. If it still doesn't work, then reboot you computer. Then it should...
I don't think it matters whether you fight Bowser before getting all the stars or not. As long as you have 120 stars when you defeat him, you'll get Luigi.
Try installing an antivirus if you can, otherwise you will need to identify the file first, and then Search for the CMD in the start menu, press right click on...