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Even with XP you should turn on the Windows firewall. If your network connections say you are firewalled, it may already be on. You usually can’t see your ISP or router firewall from inside your computer. Since XP is quite old, how long have you been using Avast? Or could there have been another AV before you got the computer? How are you connected to the internet if not through the router?
If you have been having other problems with Avast! installed, definitely worth a separate thread in the Virus section. A rootkit and multiple viruses would be very unusual with any up to date AV, so you should explain your problem to some of the virus experts there. Might also have been a false alarm, so ???