Security Certificate

Unfortunately, you didn't give me two critical snippets of information in your question: your operating system and what Web browser you're using, but I somehow suspect that it's Internet Explorer on Windows anyway, so let me answer for that configuration and see if helps you out...

Certificates are supposed to prove that you're connecting to the site you think you're connecting to, something that's particularly important if you're about to share sensitive data with the site or download a program or system patch. I won't even talk about phishing and how that ups the ante of site verification (though you can read about it here: Paypal and eBay alerts: legit or phishing?)

Since having a site certify itself doesn't make sense -- because the site could just as easily have a forged security certificate -- there are third party Certification Authorities that act as what security folk call 'trusted third parties'. It used to be that there were just two or three of these authorities, but now there are more, and what's happened in your situation is that you've somehow ended up indicating to your Web browser, Internet Explorer, that you don't trust the third party certification authority that a particular site is using.

But don't panic. This can be fixed and pretty easily at that. Go into Internet Explorer then choose Tools -> Internet Options, then click on the Content tab. The middle of that window will show you different options regarding Certificates. Click on the middle button, labeled "Certificates..."

you was security certificate on computer every time and you use internet to security certified your computer and you install in security softwares