My Win XP File and Printer Sharing in the above-described
3-computer wireless home network are now working when McAfee Personal Firewall 10.0 on PC2 has the following settings:
Internet & Network Configuration > Firewall protection is enabled (ON) > Advanced > System Services > Windows File Sharing (NETBIOS) Ports 137-139 (checked) > Edit > Open port to a computer or network that is: "Standard (includes Trusted)
If the last radio button is set to "Standard (includes Trusted)", then the sharing works as long as the range of addresses listed in Internet & Network Configuration > Firewall protection is enabled (ON) > Advanced > Networks > is set to Trusted, i.e. If the router is dynamically allocating a set of IP addresses for the network, then that range of addresses should be set here to Trusted [this was mentioned in MartyD's opening post].
If some addresses are set to Public (no longer the case in my network), then the Open port setting would have to be changed to "Trusted, Standard, and Public". The source of my network problem was a mismatch in these settings.
Also, the sharing works with the latest version of FireFox installed as an alternate browser to Internet Explorer 7 on PC2.
Conclusion: McAfee works for me with Win XP sharing in a mixed Security environment.