You have been advised to change your passwords as your email accounts may have been hacked. However, this may not be the case and your email address may have been "borrowed" randomly, which has happened to me in the past with other providers.
I am sorry you have received one of these letters it can be quite disconcerting however I would like to assure you it is not about selling you software. Basically in your case we have noticed people/robots logging into your account from around the world using your password, they are then sending spam email from the account. We have to put a stop to that and need to inform you of it. We do not know how they obtained your password but best guess is either they either guessed it or perhaps you have malware on one of your devices harvesting it and telling them, we simply do not know so we do our best to advise you accordingly www.virginmedia.com/offspam