Hi,
You can make your own digital signatures or buy them from companies such a
VERISIGN (hope I spelled that correctly).
From the help system:
Because a digital certificate (digital certificate: Attachment for a file,
macro project, or e-mail message that vouches for authenticity, provides
secure encryption, or supplies a verifiable signature. To digitally sign
macro projects, you must install a digital certification.) you create
yourself isn't issued by a formal certification authority, macro projects
(macro project: A collection of components, including forms, code, and class
modules, that make up a macro. Macro projects created in Microsoft Visual
Basic for Applications can be included in add-ins and in most Microsoft
Office programs.) signed (digital signature: An electronic, encryption-based,
secure stamp of authentication on a macro or document. This signature
confirms that the macro or document originated from the signer and has not
been altered.) by using such a certificate are referred to as self-signed
projects. Certificates you create yourself are considered unauthenticated and
will generate a warning in the Security Warning box if the security level is
set to High or Medium. Microsoft Office will only trust a self-signed
certificate on a computer that has the private key for that certificate
available (generally only the computer that actually created the certificate,
unless the private key is shared with other computers).
To learn more about certification authorities that offer services for
Microsoft products, see the list of Microsoft Root Certificate Program
Members.
Note The hyperlink (hyperlink: Colored and underlined text or a graphic
that you click to go to a file, a location in a file, a Web page on the World
Wide Web, or a Web page on an intranet. Hyperlinks can also go to newsgroups
and to Gopher, Telnet, and FTP sites.) in this topic goes to the Web. You can
switch back to Help at any time.
In Microsoft Windows XP, on the Windows Start menu, point to All Programs,
point to Microsoft Office Tools, and then click Digital Certificate for VBA
Projects. In Microsoft Windows 2000, on the Windows Start menu, point to
Programs, point to Microsoft Office Tools, and then click Digital Certificate
for VBA Projects.
If the SelfCert.exe file is not on your computer, you might need to install
it.
How?
Run the Microsoft Office Setup program again.
How?
Quit all programs.
Double-click the Add/Remove Programs icon in the Microsoft Windows Control
Panel.
Do one of the following:
If you installed your Office program as part of Microsoft Office, click
Microsoft Office in the Currently installed programs box, and then click the
Change button.
If you installed your Office program individually, click the name of your
program in the Currently installed programs box, and then click the Change
button.
On the Advanced Customization screen in the setup program, click the expand
indicator (+) next to Office Shared Features.
Select Digital Signature for VBA projects, click the arrow next to your
selection, and then click Run from My Computer.
Follow the instructions in the Create Digital Certificate dialog box.
There is much more in the help system you might want to look at. The above
is for 2003.