This is the reason why I stopped using ZA PRO. For the love of god, a new update and a system crash. Not to mention, that it would block my internet connection if I didn't open a new browser window every 2 minutes. Hell of a way to lose 1h of browsing time
I now use KAV and will use KIS after we move house. I haven't had a problem, maybe once in a while and with a half-stable release (then the kind forums guys pointed me to a more stabile version).
I myself recommend KIS, sure it's expensive, but the extra bucks, means you won't be spending them on Aspirin for your ZA headaches.