I moved this to Malware Discussion just in case. Check your sent mail boxes and if nothing in there then it wasn't you.
There are certain worms that infect Windows and can spoof email addresses for the sender from someone else's address book as spam emails. It could have originated on any machine belonging to someone who has your email address.
I've had this happen and been blamed similarly to you but the emails never went out from my machine. Run Stinger and Malwarebytes Free, both linked in the last item in my signature below and suggest to any friends they do the same.
So yes, it's normal for the scans to turn up nothing on the clean machines, but someone, somewhere has a problem. In my experience it's usually a commercial enterprise somewhere. In my last case it turned out that Amazon.ca client email base had been hacked.
They eventually realised that and notified all customers to change their passwords.