T-bird & failed email encryption

Just started using Comodo and decided to jump in a find out what email encryption is all about. Not only got my feet wet, but my head as well. I got and imported a cert, then as soon as I started trying it out, every email I sent showed this;
Thunderbird cannot decrypt this message

The sender encrypted this message to you using one of your digital certificates, however Thunderbird was not able to find this certificate and corresponding private key.
Possible solutions:

* If you have a smartcard, please insert it now.
* If you are using a new machine, or if you are using a new Thunderbird profile, you will need to restore your certificate and private key from a backup. Certificate backups usually end in ".p12".

I then deleted the cert, made sure that sending and receiving/signing and encrypting check boxes are clear, restarted and even rebooted, but this message keeps coming up. How do I completely remove/reset/restore to before the cert?
Windows XP Pro, 5.1.2600

OK, I missed one post when I did my search. Went into SecureEmail Configuration and set levels low enough to prevent including anything about the certs in emails out or in, but still, what went wrong? I’d like to be able to use the cert feature, but for my friends who don’t use any such security yet and are as dumb as I as to how it works, how can I use it or keep it in the background until it becomes ‘fashionalble’?

Hi, FBClark

As i understood, “every e-mail I sent” should be “every e-mail I receive”, because Thunderbird can not decrypt the encrypted message, So you need to install your own certificate also in Thunderbird store by following steps:
Open Thunderbird:

Tools->Options->Advanced-> Certificates tab → View certificates → press Import button, choose your Pfx file (your cert) and finish importing.

Then, restart Thunderbird and all should be OK.