I have a put up an initial hit parade of irritating software based on the first 100 repsonses (a more in-depth analysis of all results to be published shortly).
In first place, perhaps unsurprisingly, are antivirus programs. I've used practically every major antivirus program on the market, and have found every single one to be completely infuriating. Whether it's popping up irritating paranoic messages every few milliseconds, making it difficult to access certain web sites properly or slowing down your eight-core CPU to the speed of a ZX Spectrum on tranquilizers, every single antivirus product appears to be designed to make you want to throw your PC off a very high balcony.
I was somewhat reassured to find iTunes in second place. Especially under Windows, I have also personally found it to be one of the most buggy and infuriating pieces of software ever written. But whether it's freezing every five seconds, launching into a lengthy "sync" process just because I accidentally wanted to upload an extra song or thinking at random intervals that my iPhone has mysteriously transformed into a new one never seen before in the history of humanity, I now feel less alone in the universe knowing that other users share my view.