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]
Since Microsoft ISA 2000 Server does not have a direct interface for generating Certificate Signing Request, you may need to follow the CSR generation instructions for Microsoft IIS4/IIS5/IIS6 web servers [2]
Links:
[1] http://www2.digi-sign.com/about/announcements/2048
[2] http://www2.digi-sign.com/support/digi-ssl/Microsoft+iis+5+iis+6