Open the folder which holds the certificates you have downloaded.
Right-click on the certificate you wish to install, and select Install Certificate from the context menu.
Click the Next button in the Certificate Wizard welcome window.
Click the Next button in the Select a Certificate Store window.
Click the Finish button in the Complete the Certificate.. window.
You may be asked to confirm the root certificate installation. Click the Yes button if a window like this appears.
Windows confirms the root certificate was successfully imported.
You may now install your personal e-mail certificate.