On Windows Server with IIS you typically import a PKCS#12 (.pfx) file that contains the certificate, chain, and private key together. If MixSSL supplied separate PEM files, you can use IIS Manager or the Windows certificate store import wizard, or convert formats with the MixSSL SSL Converter tool to build a .pfx (you will need the private key and a password you choose).

1. Open IIS Manager → select the server → Server Certificates.
2. Choose Import for a .pfx, or Complete Certificate Request if you submitted a CSR generated on the server.
3. Bind HTTPS on your site: edit site bindings, add port 443, select the imported certificate.
4. Browse to your site with HTTPS and confirm there are no certificate warnings.

For managed Windows hosting, follow the host’s SSL article—they often ask for the certificate and key separately.