StartCom/StartSSL SSL certificates cannot be installed on a Windows Server because Microsoft no longer trusts them as valid.
This isn't a Microsoft-specific issue however. The three major browsers (Chrome, Safari and Firefox) have also removed support for the certificates. They were removed from Firefox in October 2016 and Google Chrome in March 2017, and that included the removal of certificates previously issued.* It's expected that removal from most lesser-used browsers will follow.
Trust was dropped for StartCom certificates when it was found that StartCom had issued a number of improperly validated certificates at a time when ownership of the company changed hands.
With StartCom certificate support removed from most of the world's web browsers, they are of very little value, so it is understandable that Microsoft took steps on Windows Server to revoke the trust of the certificates.
*While Startcom certificates issued before October 21st, 2016 may still work in some older versions of Chrome and Firefox, that support is expected to be discontinued sometime in 2017.
Note that you may also see the same issues with SSL certificates from WoSign - that is the name of the company that bought StartCom.