Microsoft Knowledge Base Article 329508 contains the following summary:
This step-by-step article describes how to install a server certificate that you have obtained from a certification authority (such as VeriSign or Thawte)
after you have accidentally deleted a pending request for the certificate in Internet Service Manager.
This problem occurs when you generate a certificate request, but the pending requested is deleted before the Web server certificate that you have received from the certification authority is installed. When this occurs and you click Server Certificate on the Directory Security tab of that Web site to install the certificate, the Process the pending request and install the certificate option is not available, and you cannot process the pending request and install the certificate.