Just been looking to install a SSL on my Win XP Pro IIS 5.1 and came across at Microsoft

If you are using IIS 6.0 and need to install a certificate for testing or internal use, you can do it in about 2 seconds with the use of the SelfSSL utility from the IIS 6 Resource Kit. Just type SelfSSL at the command line and it will instantly install a self-signed certificate on the default web site. You can install a certificate on other sites and control the contents of the certificate including the common name and expiration date using command-line switches. See the SelfSSL documentation for exact details.

If you are not using IIS 6 yet, then you can use the SSLDiag tool available from the Microsoft Download Center. Be sure to download the full setup.exe package in order to get the SSLDiag documentation. To install a certificate on a web site, right-click the Web site level (shown by [W3SVC/]) in the main window of SSL Diagnostics, and then click Create New Certificate. This tool does not give you the ability to customize the certificate, but does allow you to install an automatically-generated certificate - intended to be used for testing purposes

You can find the original page with all this and more here