I received an email, too. WebSafe is always on, McAfee anti-virus is always running and the firewall is always on. I ran a full scan and no threats were found.
It's all very well Virgin Media telling me that, although it "probably [wasn't my] fault", my account could be cancelled due to their Acceptable Use Policy but they have given me no real help in preventing the attack or in finding or resolving the problem. (They told me to protect myself in the future by using WebSafe - as I've said, I do use it. If the tools they give us aren't up to the job of protecting us, surely the blame lies with them?)
It's a comfort to me that other people have also had such emails. Hopefully, as someone said above, this is just an erroneous message. If my network really has participated in a DOS attack, I don't see how I can prevent this from happening again without help from Virgin Media.