1. Click on the 'Tools' menu and go to 'Internet Options' from the Internet Explorer window.
2. Click the 'Content' tab and then click the 'Certificates' button. Keep clicking the small arrow on the upper right side of the dialog until you see the 'Untrusted Publishers' tab. Click that tab once you see it.

3. Click on any entries that are possibly linked to your problem and click the 'Remove' button. This will let Internet Explorer trust the certificate once again.