Maybe you set msn.com as your home page by accident? Then choose another one. http://www.google.com/support/chrome/bin/answer.py?answer=95314 tells how to do that.
I usually use Chrome as my browser however when I logged on tonight MSN appeared as my browser, even though I went to the Start Menu and "opened' Google Chrome. Is this some new 'magic' from Microsoft? Sometimes though when I click on the tab behind the MSN page Chrome is behind it. But I don't want the MSN page at all, so how can I prevent it from appearing?