Email received containing my old password

I have received an email containing my old password and think this is a result of a breach on a website that I was registered on. However, I cannot be sure and it is unclear how they associated the email address with the password because the email address from which I received the email is not often supplies online to access websites.

I have Comodo Internet Security 2018 installed and scans show nothing. How can I be sure that there is nothing malicious on my machine? I have run a number of other free scanning utilities and none of them are showing anything but I still feel uneasy.