Welcome to the Forums mate!
Kudos on running Windows 2000 for this long. I'm pleased to hear that.
As far as I know ZoneAlarm would not work as freeware. (But I have not tested it so I can't tell for sure.)
Alternately you can try Old Version of Avast Free Antivirus 6.0.1270 Download - OldApps.com which I run on my Windows XP and it works flawlessly.