You’ll never know if your PC has been zombified unless you check. IT Observer gives you a few clues, but I will make it even simpler.
Install a free copy of the ZoneAlarm firewall (and turn off the Windows firewall for a bit as well). Keep ZoneAlarm access messages on, and wait. If you start seeing all kinds of popups coming from your taskbar, with no applications active, that is likely the zombie talking. Those popups signify your computer trying to access the internet every which way from Sunday, with spamming and further infection attempts high on the priority list.
If you are an everyday joe, you can pick up some anti-virus tools to clean things up. If you are a Microsoft executive, you just reformat your hard drive and start over.