Were there any other AV installed on the w2000 system if so what and how was it removed ?
Do you have any other security software that make stop changes to the registry/startup folder, etc.
Run ashDisp.exe that should display the icon on the tray as a work around, whilst the issue is resolved.
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
If that doesn't work try, uninstall, reboot, install, reboot.
It would probably be best to download the latest version of avast and save it to your HDD, somewhere you can find it again. Use that when you reinstall.