When a browser encounters a site that has the "https" prefix (meaning a 'secure' connection), that server presents a 'certificate' that is telling the browser they are the genuine site.
The browser in turn checks several items on that certificate to verify it's real, and valid.
One of the things it checks is it's validation time-frame ('from' so and so date; and 'to' such and such a date). If that does not match exactly what it should be, the browser throws up a warning.
For whatever reason, Windows computers can loose track of the correct time & date, so the certificate does not coincide.
Most common fix:
Right click tray time> "Adjust..."> Update.
While here, tick 'Automatic synchronize...'
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While you're at it, verify the site has fixed any 'heartbleed' issues (the recent web incident) by using this utility: https://www.ssllabs.com/ssltest/index.html
Real e-z, and takes about a minute.
If it has not fixed the problem, beware that there could be a leak of information with serious consequences.