You are logging in incorrectly; you should not include = in your login. So to log in, type:
mysql.exe -uroot -padmin
If that doesn't work, then you may not have your system configured. If so, then here's a good tutorial on getting started with the MySQL prompt: http://breakdesign.blogspot.com/2007/11/getting-started-with-php-and-mysql-in_11.html