In your case that may be true. But are you sure?
I wish I could tell you how many times I have heard that from somebody that wanted me to do something to their computer, upgrade or whatever, and found multiple vermin running around inside it. That is especially true of Windows installs that are owned by a bot net or are being used for some other reason that the infection wishes to remain silent. Anti-virus runs just fine, is updated, and is as happy as a dead pig in the sunshine.
You hook it to the internet and hang a packet sniffer on it and you find all kinds of strange and unusual activity taking place.