Effective 1 January 2011, we will no longer support any Certificate Signing Request [CSR] generated with a 1024 bit key. This is because NIST, PKIX, WebTrust and other respective security standards no longer consider the 1024 bit key size as secure. Read more > [1]
The best resource for Microsoft OCS 2007 is to go directly to the Microsoft TechNet site and follow the instructions for sub section 3.6 Configure Certificates for Front End, Web Conferencing and A/V Server Roles [2].
Once you have followed these instructions, then visit sub section 3.7 Configure the Web Components Server IIS Certificate [2].
Links:
[1] https://www.digi-sign.com/about/announcements/2048
[2] https://www.digi-sign.com/http