Personal firewalls (XP's Internet Connection Firewall [ICF], Norton Internet Security, ZoneAlarm, etc.) monitor (and block) all network traffic in and out of the machines they run on, which usually includes file and printer sharing requests.

The first stop in this category is to make sure that ICF is disabled on every computer on your LAN. ICF is pretty basic, i.e. non-configurable, and only intended to be run on machines directly connected to the Internet. If it's enabled on computer that are behind a hardware router's firewall, that machine is guaranteed to have problems with File and Printer sharing. Figure 6 shows where ICF is located and how it should look when disabled.

Figure 6: Internet Connection Firewall disabled

Most other personal firewalls are configurable and can be set to let Microsoft Networking traffic pass without problems. But if you don't know how to do this, you better disable, or better yet, uninstall them until you get file sharing working.