website’s security certificate?

[infiSTAR.de]

Check your system's clock.  If your CMOS battery goes dead and your clock goes back to a default (like 1999 or 2002) then all certificates will be invalid because your computer's idea of when "now" is will predate the actual release date of the certificate, so it won't yet be valid.

Source: http://answers.microsoft.com/en-us/windo...e1c?auth=1