Having successfully signed up for the certificate I received the collection mail and clicked on the install button. So far so good but look at the attached screenshot to see what happened (this message will not go away).
My PC is protected by Comodo Internet Security. Could that be the cause?
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What e-mail client or browser? Thunderbird didn’t work for me so I claimed it via Google Chrome and exported it with the private key and then imported it into Thunderbird and that worked, you might have to do the same.
I use Firefox. But the issue is that the certificate provided cannot be verified. I want to use it with Outlook 2007. Is there some way to import it into Outlook?
If I remember correctly you should have received a few e-mails, in one of the e-mails there should be a link to a Comodo page that goes through how to install it in X e-mail client and also exporting the certificate from X browser/e-mail client and importing it into X e-mail client.
Edit:
Please visit http://www.comodogroup.com/support/products/email_certs/index.html for guidance on configuring your email client to use your certificate to secure email.
Note:- We strongly recommend that you export your certificate to a safe place in case you need to reload it later. For details, please see http://www.instantssl.com/ssl-certificate-support/server_faq/ssl-email-certificate-faq.html.
Thanks Sanya, I will look into that immediately.
Edit: that page is at least 10 years out of date. The latest version of Outlook it provides instructions for is 2003. 
Hmm I’ve never used outlook so I don’t really know how to do it.
My suggestion is to try and get the certificate through Internet Explorer, Google Chrome or Comodo Dragon and then follow this link for instructions on how to export the certificate and then follow this link for instructions of how to import it into Outlook 2013.