2 issues are involved:
Certificates:
You could try a couple of things:
Odd as it sounds, your systems' time must be synchronized to the actual time. Although your machines 'on-board' certificate library may be current, the machine's time is not correct, rendering a mismatch for the sites certificate parameters.
Right click the system tray time clock> Adjust time & Date> Internet time> Update now. (Make sure your system is set to 'Automatic Synchronize...' while you're here)
Another possible fix (XP):
With Internet Explorer, go to Microsoft Updates, then select the 'Custom' button (not the 'Express'). That will tell you all specific updates for your computer (hardware & software) and in this case the certificate updates.
Note: persistent time malfunction could mean malware problem or a motherboard battery issues.
Also of significant import, make certain to have this configuration in your browser:
Tools> Internet options> Advanced> Security heading: check "Warn about certificate mismatch" OK out.
This is really critical to prevent spoofing of SSL certificate issuance, potentially showing the lock and "https://" when in fact it is NOT the real McCoy.
Secure content advisory:
This may relate to a setting in your browser.
It is intended to alert you whenever there is both encrypted and non-encrypted displays on a webpage; in the event you were under the impression it was completely 'private'.
It may merely be a case of some 3rd party advertising (or other "asset") that is not encrypted, and usually does not mean there is a serious problem.
To circumvent these issues in the future, make the following changes:
FIREFOX: Tools> Options> Privacy: Check 'Accept Cookies from sites'
& Un-Check 'Accept 3rd Party cookies'.
And: Tools> Options> Security> Warning messages heading-Settings: on the next window, make selections based on your 'comfort level' and understanding of site security, and so on> OK> OK
INTERNET EXPLORER: Tools> Internet Options> Privacy> Advanced:
here check 'Override automatic....'; 'Allow session cookies'; 'Allow 1st party cookies' & 'Block 3rd Party Cookies'.
Tools> Int. options> Advanced> Security heading: uncheck "Warn if changing between secure and not secure..." > OK out
Spl. note: Also in this section, make sure to have 'Warn about certificate address mismatch' which helps curb 'spoofed' certificate sites.