I have the same letter, and the same as for you, it's threatening to disconnect my broadband if I don't sort this out. I'm not an IT boffin who knows the ins and outs of these kind of things.
I did an extensive and very long scan last night to remove anything that could possibly be lurking. I've followed the guidance re looking up to see if there's anything being forwarded on my Virgin Media email and its drawn a blank - Nothing.
I'm now in the process of changing all my passwords.
Like you, I don't appreciate the tone of the letter. It implies that I'm a cyber criminal, and after all these years of being a Virgin customer I expect a little more respect - and assistance!