Cannot access web sites, certificate error

I keep on getting the message.

Revocation information for the security certificate for this site is not
available. Do you want to proceed?
Yes No View Certificate
This happens mostly on web sites that I need a user name and password. The only change I have done was installing Yahoo Toolbar for internet explorer. I removed it thinking that it is causing the problem but I am getting the same thing. I also regularly clean my computer by deleting temporary files and cookies and running Ccleaner. I also tried scanning the computer using AVG 9.0 but found no virus. Any help will be appreciated.
Patricia Jameson
