No, I didn't put another firewall on the PC, just put Avast Home on the PC. I have Windows 2000 on this PC.
avast isn't a firewall, which is an essential pert of your system security.
avast does have the Network Shield, but that is by no means a firewall, it is looking at areas that are usually exploited by the likes of Sasser/Blaster, etc. You need a firewall.
Installing a firewall is unlikely to resolve the problem with AIM as Neal said, but without one you could have more urgent and serious problems without it. The time on-line for an unprotected system to infection is counted in minutes and a firewall is an essential element of that protection.