ICF (XP's own firewall) is optional and more. I won't duplicate the web but why not look at ICF in more detail or just install a firewall today?
When I turned my desktop on after returning from a trip, the antivirus program started catching Trojans and put them in quarantine. I then scanned with Malwarebytes and it identified more Trojan viruses. When I checked the Windows Firewall, I get the message "Due to an unidentified problem, Windows cannot display Windows Firewall settings." I searched all the Microsoft community and support sites and found numerous users identifying the same question concerning the firewall. None of the suggestions worked. I also ran Microsoft Baseline Security Analyzer and it stated "Windows Firewall not installed". When I checked the Windows Folder on my hard drive, under System32, the firewall applications were not there.
Desktop is Dell XPS 650, Windows XP Professional, SP3. It was completely disconnected while we were gone and the firewall was always turned on.
Laptops connected to my home network are Windows 7.