Chaps,
I'm afraid this thread contains some dubious advice. You are taking a significant risk if you ignore this particular security warning because it is reporting a problem with the digital certificate that proves that a website really is who it claims to be.
There are a number of 'legitimate' faults that can cause the warning, most likely being that the certificate has time expired. But the purpose of the certificate, which is part of the electronic security system and not just a bit of paper, is to protect you and the site from spoofing. The warning may well mean that the website's address has been hijacked and that you have been misdirected to something only pretending to be trustworthy. Such as criminals masquerading as your bank or a favourite shop.
Under no circumstances whatever should you spend money on a website with a certificate problem; nor should you use it for downloads or give it any personal information. Wait for the site owner to restore trust by fixing the underlying problem before you do any business with it. It's safe when you don't get that warning message.
Ignoring this type of security warning is analogous to weighing down a safety valve because it is "leaking". You'll probably feel very foolish if you get blown up because the real problem was a stuck pressure gauge…
Cheers,
Dave