The problem is that someone could be performing what's called a "man-in-the-middle attack."
In order to be able to read your secure traffic, they've substituted their own digital certificate.
I suspect that your system is infected with a virus. I recommend that you shut that machine down and take it to someone who can perform a security scan.
Meanwhile, log in from a different computer and change your passwords (especially to GMail!).
Typically, the goal is to get access to your E-mail account, and then try to send password reset messages from banks, online retailers that might have your credit card information, etc. in order to get access to those accounts.