Nastiest option, of course. Windows Update it's not exactly a big offender, as it downloads the updates whenever I instruct it to do so (that means I refrain myself for doing something during that time period), but other software... OK, my main problem is not software stealing focus, as I don't remember it happening too often (they flash instead, that's a much better idea), but particular implementations in some software or Internet services.
Why the f****** GMail tab has to steal the focus of what I'm reading at the moment? Why IE can't pop up a nifty dialog at the top of the window likes it does with most warnings when a page wants to install something?
Considering the rather excellent implementations other software (popup notifications like Firefox, Miranda or Opera), or the same Windows (tooltips in the systray, that's fantastic!), I can't figure out why some others insist in stealing focus prompting for your intervention and (possibly) starting a disaster in your computer.
If you really have to popup a window, make sure that you use ONLY the mouse to interact with it (no keyboard keys allowed), and set as default the less dangerous option of the dialog. Think in antivirus software nag screens when they encounter a virus in your computer, practices like these should be de facto in every security software, for the havoc or lack of it they could unleash on the machine.