2-PC LAN + network printer + router

Win XP Pro SP3

Can Windows Firewall affect / stop LAN traffic? The following problem often
raises questions about the firewall and I want to understand its relevance.
Discussions seems to get very convoluted and end up down blind alleys.

I contend that Windows Firewall (WF) only affects WAN traffic but I suspect
that just by replacing router#1 with router#2, this does not result in the
WF stopping PC-PC ping traffic. Is that right?



I am trying to replace a router with a new one. With router #1 in place Ping
between the PCs works fine. With router#2 in place all network traffic is
fine (eg PC-printer, PC-router & PC-Internet) apart from Ping PC-PC.

I have been through the usual issues of network configuration issues
ad-nausium includin IP addresses, DNS, sub-net mask etc. In deed if there
was a problem, then nothing would work with router#1 in place. But with
router#1 in place all network comms is fine. Put router#2 in place and all
network traffic is fine apart from ping PC-PC.