If you are thinking of updating to ZA you might like to wait a while until the UI problem is fixed.

What happens is your startup takes ages and ZA reports that protection is enabled and it is trying to "initialize UI" It remains like this permanently, and keeps flashing a UI initialization pop-up. Your internet browsing will also probably slow to a crawl.

The solution that I have found was to download ZA for later installation, boot into safe mode and completely uninstall the old version. Also uninstalling the Anti-Spyware thingy.

Boot into normal mode and install without the anti-spyware toolbar. You should now be able to access the UI.

Not sure what causes this but it does not seem to be restricted to the free version, and apparently affects both XP and Vista installations.