I had been using the old version of Zonealarm quite happily on XP for quite a while, and then they emailed me and recommended that I upgrade to the latest version.
From the moment I installed it, I had horrendous problems. It kept crashing my machine, I tried erasing all the old settings, and reinstalling it several times, but when it nearly trashed my hard disk, I gave up and emailed their tech support. Despite contacting them several times, after a fortnight I still had no reply, so I switched to 'Sygate Pro 5.0' which works perfectly for me. The interface is not as flashy as Zonealarm tried to be, but in my opinion it is better. The only cute feature I missed was the renaming of email attachments.
Amusingly, over a month later Zonealarm tech support finally contacted me, but I told them that they were too late, and their definition of 'support' was certainly not the same as mine.