Date and time is the most common reason I could find. here's a couple more things I found:
1. In Windows Internet Explorer 7, click Continue to this website (not recommended). 80% the issue would be resolved once you follow this step.
If not, you will see a red Address Bar and a certificate warning appear, keep clicking on next & cu will get a PHISHLING FILTER. Click on "YES", close ie and re- open.
2. Another cause for this issue is that the computer's time/ date could be wrong, because in that case, the certificates may show up as expired. Change the computers time to a different date and month and revert back to the old one (Believe me, it works !!!)
3. In rare scenarios, resetting modem & for Windows Vista users, right clicking on Internet Explorer icon, "Select Run as Administrator." has worked.http://social.microsoft.com/forums/en-US/whssoftware/thread/a83f19e9-e4b1-46ba-905a-92672ccdfa93/