About a month ago Back in February, I posted a fix to the problem of not being able to log in to your WordPress admin panel if you are running IIS as your web server. That fix has been superseded by the excellent work of the WordPress folks. I have created a zip file with all the necessary edits made to the required files. You should be able to simply download the zip file and extract them over your current 1.5 install.
The fix is now easy. Download version 1.5.1!