Safari can't verify the identity of the website

This message comes up every time I try to log in to my bank, ebay, paypal, vodafone, hotmail, etc. Basically everything I try to log into. It tells me the certificate is invalid, or the issuer is invalid. This just started one day a little while ago. I have all these pages bookmarked and safari used to be fine with them. I downloaded firefox to see if it was just a safari thing, and it did not have this problem. Can someone please tell me how to fix this with safari? Version 5.0.1