Second Life gives me an error message about being "Unable to find a valid certificate" or "Unable to establish a secure connection to the login server."
Your local network may be blocking secure logins. It's also possible that your secure login is failing due to an incorrect date/time on your computer.
Try double-clicking the clock in the Windows system tray (in the lower-right corner of the screen), and verify the date and time. Make sure it's not a year off!
Be sure a firewall, or internet security or firewall software, is not blocking Second Life access.
If you are on an office or university network, your network administrator may have secure logins disabled. In this case, you may not be able to use Second Life on that network connection.