Security certificate problem

Ever since I updated XP a few weeks ago; a window keeps popping up every time I log into Hotmail or other sites that require a password. The window says that the security certificate has expired or is not yet valid. This is not true as the certificate dates are valid. When I use the wizard to validate the certificate; it only temporarly fixes the problem. The next time I try and log on, the same thing happens all over again. Help will be greatly appreciated.