The first time I came across this issue was when a guy called me because, after re-installing WinXP on his computer, he kept receiving the following error message when trying to get online: the security certificate has expired or is not yet valid.
Tried updating the certificates according to info on the internet, then tried rolling Internet Explorer back to the previous version. Still nothing.
I got him to check out his settings and compare them to mine. I opened IE, then clicked on Tools, Internet Options, then the Security tab and clicked on the Custom level button.
Down towards the bottom is a “Reset custom settings” option, mine was set to Medium-high (default) his was on High but he was unable to change it.
We went step by step through all the settings in the list above trying to change it manually, using the settings on mine as a guide. But when he clicked on OK to save the settings the certificate error message still kept popping up.
So online again, finally coming across this simple solution. CHECK THE DATE ON YOUR COMPUTER. Browsers have security certificates built in that have a start date and an end date. If the date on your computer is off one way or the other, that can cause this error message.