The levels of security to help reduce macro virus infection are as follow:

• Very High Only macros installed in trusted locations will be allowed to run. All other signed and unsigned macros are disabled. You can disable all macros entirely by setting your security level to Very High and disabling macros installed in trusted locations. To disable macros installed in trusted locations, click Tools, then select Macro and Security, and then click on the Trusted Publisher tab and “uncheck” the Trust all installed add-ins and templates option.

  Note  This also disables all Com Add-ins and Smart Tag .DLLs as well as macros

• High You can run only macros that have been digitally signed and that you confirm are from a trusted source. Before trusting a source, you should confirm that the source is responsible and uses a virus scanner before signing macros. Unsigned macros are automatically disabled, and the file is opened without any warning.
• Medium A warning is displayed whenever a macro is encountered from a source that is not on your list of trusted sources (described below). You can choose whether to enable or disable the macros when you open a file. If the file might contain a virus, you should choose to disable macros.
• Low If you are sure that all the files and add-ins you open are safe, you can select this option— it turns off macro virus protection. At this security level, macros are always enabled when you open files.

By default, the security level is set to High. If the security level is set to Medium or High, you can maintain a list of trusted macro sources. When you open a file or load an add-in that contains macros developed by any of these sources, the macros are automatically enabled.

A digital signature on a macro is like a wax seal on an envelope— it confirms that the macro originated from the developer who signed it and that the macro has not been altered.

When you open a file or load an add-in that contains a digitally signed macro, the digital signature appears on your computer as a certificate. The certificate names the macro's source, plus additional information about the identity and integrity of that source. A digital signature does not necessarily guarantee the safety of a macro, so you must decide whether you trust a macro that has been digitally signed. For example, you might trust macros signed by someone you know or by a well-established company. If you are unsure about a file or add-in that contains digitally signed macros, carefully examine the certificate before enabling macros or, to be even safer, disable the macros. If you know you can always trust macros from a particular source, you can add that macro developer to the list of trusted sources when you open the file or load the add-in.

If you are a developer, you can digitally sign macros from within the Visual Basic Editor (Visual Basic Editor: An environment in which you write new and edit existing Visual Basic for Applications code and procedures. The Visual Basic Editor contains a complete debugging toolset for finding syntax, run-time, and logic problems in your code.).

When you open a file that includes signed macros, you are prompted whether you want to trust all macros originating from that publisher. If you select this option, you add the certificate's owner to your list of trusted publishers. Before you decide to do this, you should review the details of the digital certificate— for example, look at the Issued to and Issued by fields to determine whether you trust its publisher, and look at the Valid from field to determine if the certificate is current. The certificate may also include details such as the e-mail name or Web site of the person who obtained the certificate.

Once you add a person (or corporation) to your list of trusted publishers, Office will enable macros signed by this trusted publisher without showing you a security warning. However, it is possible to remove entries from the list of trusted publishers.

Note  Any certificate trusted in the list of trusted publishers will also be trusted in Internet Explorer.

When you open a template or load an add-in from the Startup folder, macros within the file may be automatically enabled. However, you can clear the Trust all installed add-ins and templates security option to receive a warning about these macros. The warning will vary according to the level of security you have chosen.