I got these instructions from our certificates provider, which I used to install them (.crt, with the intermediary files) on linux Apache servers. The drill should be exactly the same for windows, except you'd use windows paths (e.g. c:\apache\certs to keep them in):
https://support.comodo.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=1203&nav=0,1,95
Hope that helps.