As always, it is important that you be very sure that the website is legitemate, if you choose to continue browsing the webpage. Also, you may report the certificate to report online to heise server.
Click here to download and install Heise SSL Guardian. The add-on works on Windows Vista/XP and earlier on Internet Explorer 7 and earlier. Once installed, Heise SSL Guardian will open a test page with a bad certificate to test if it works OK.
If you are a Firefox user, ensure you update to Firefox 3.0 and follow this article here to install Perspective & SSL Blacklist extensions to protect yourself on HTTPS websites.