Even worse; sometimes the virus scanner can be an even bigger problem than the threat its supposed to stop. When I started doing more company stuff on my PC (self employed) I decided that since I liked Avast up to that point that I should simply show some support and apply for a one year subscription.
And then it started; they introduced their "Internet security suite" and I got a free upgrade. It could scan my e-mail, web traffic, the system itself and all through separate engines. So far, so good. Since I don't use torrents / peer to peer stuff on this PC I could turn that down, messenger and such; same deal.
However; I soon started noticing that whenever I did a global update on some in-house software (which basically opens 20 - 30 simultaneous network connections for a moment and passes a few kB's of data) then my PC would freeze. Completely. Only after a while it would become responsive again.
You never guess what it was; Avast. And not even because it thought that I had some sort of virus; because their firewall was plain out crapware: it simply couldn't cope with a simultaneous 30 peer data stream, instead it sucked up all the resources it needed to cope.
Right now I use MS security essentials, the PC gets a full scan every once in a week and that's the end of it.