First copy the cert from the server to a file. Log into the server as administrator, and in CMD:
certutil -ca.cert ca_name.cer
Then copy ca_name.cer onto the desktop of the client PC, double-click on it, click “Install Certificate…” on the General tab, click “Place all certificates in the following store”, click “Browse…”, click “Trusted Root Certification Authorities, and click OK. Click Next and Finish.