1. Click Start, point to Settings, click Control Panel, and then double-click Internet Options.
Or, start Internet Explorer and select Tools, Internet Options.

2. On the Content tab, click Personal, click a certificate you want to Remove, and then click Remove.

note: When a digital certificate is removed

• Any e-mail that is encrypted with the associated digital ID is no longer readable.
• This includes e-mail that you received before you removed the digital certificate, as well as e-mail you receive after you remove the digital certificate.
• The e-mail is encrypted using your public key, and because the digital certificate has been removed, you no longer have the private key needed to decrypt it.
• To read this e-mail again, you must import the digital certificate back into Internet Explorer, and then enabled it in Outlook Express.
• There is no method of exporting encrypted e-mail to an unencrypted format.
• If you receive any encrypted mail that you must be able to access, make sure you have successfully exported the digital certificate before you remove it.
• You may also be unable to view any Web sites that require client authentication based on that digital certificate until you either import it again, or generate another digital certificate to use for that Web site.

For these reasons, Microsoft does not recommend removing digital certificates. You should keep the current digital certificate and obtain a new one for a new e-mail account or Web site that requires one.
However, if this is not possible, and a digital certificate must be removed due to incorrect operation or for troubleshooting purposes, follow these steps:

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the 'Changing Keys And Values' Help topic in Registry Editor (Regedit.exe) or the 'Add and Delete Information in the Registry' and 'Edit Registry Data' Help topics in Regedt32.exe.
note that you should back up the registry before you edit it.

Delete all the folders under the following registry key, and then restart your computer:

When you remove the digital certificate from Internet Explorer, the associated digital ID is removed from Outlook/Outlook Express.