Good antivirus software, once installed, doesn't make its presence known until its needed -- that is, it's detected a virus, trojan or other nasty and it requires your attention. Unfortunately, while it's nice not to be bothered by unnecessary dialogs, it makes it easier to forget it's running in the background, monitoring file operations. This isn't usually a problem, unless you happen to be copying a few gigabytes of data.
Recently I was reorganising files on my desktop machine after the installation of a new SSD. At one point I set up a transfer between a pair of magnetic drives and went about my business, until I noticed that the operation would copy a few files, stop and continue again. This happened regularly enough that I couldn't put it down to the idiosyncrasies of magnetic media.
It seems obvious now, but it took a few minutes for me to realise that I had Microsoft Security Essentials' real-time monitoring activated -- as you do. After turning it off for the duration of the copy, the effect was instantaneous and noticeable.
I was always resistant to installing AV software for this reason, but the benefits of having some protection over the minor inconvenience of disabling it every so often for the odd gigantic file transfer is worth it. That said, MSE doesn't have the best track record in recent years compared to competing products, so I'm on the hunt for a replacement -- any recommendations?