'Welcome to Darwin'??

When I turn on my iMac (2007), it can properly boot up and bring me to the login screen. It doesnt matter which user account I click on, as soon as I enter the password, a black screen comes up with the words, 'Welcome to Darwin' in white. This started happening in February of 2011. I have only booted up the computer a few times since then, because I'm at a loss as to what I should do. Help please! 😀

Thanks,

Karissa