Two potential strategies to handle this.
1. Pass the username/password as part of the URL (Google will help you find the exact steps)
2. Use AutoIt. This is what we use in Watir to get past this type of issue...it should be similar in Selenium.
"The single biggest problem in communication is the illusion that it has taken place."
-George Bernard Shaw, Irish playwright and Nobel Prize winner, 1856-1950