If, the first time you log into administration, you see something similar to the following at the top of the main menu in the administration program:

... you probably also get a Bad Login error in the right frame initially or if you attempt to click on any of the links from the main menu.

The most common cause of this problem is that your Windows server is set up to run your Perl scripts through the perlis.dll library rather than the Perl executable itself. (Discus must be run through the perl executable.) The cause and solution are the same as seeing Content-type: text/html on top of all script pages (which you probably see as well).