There are many cases when you need a certificate installed on your IIS server. A typical example is SSL enabling OWA (Outlook Web Access) for achieving a higher level of security. Another very good example is configuring Forms-based authentication in OWA which requires the use of SSL.
You have two choices:
- Purchase a certificate
- Use a self-signed certificate
Most of the instructions which you can find on Internet describe the procedure for installing first your own certification authority, then requesting a certificate, and last - installing it.

Here you can see a much easier and straight forward procedure - using the Microsoft SelfSSL.exe tool. You just run it, provide the FQDN (Fully Qualified Domain Name) for your IIS server, specify the validity of the certificate, and that’s it.
The SelfSSL.exe tool is part of IIS6 resource kit. You can get just the "SelfSSL.exe" tool from here

NOTE:If you are looking for installing Self-signed certificate in IIS7, Windows Server 2008 – check the following Screencast:
How to Install Self-signed Certificate – IIS7, Windows Server 2008