If the connectivity between the machines is suspect I would absolutely start there.
My first guess is the pc’s have an os-level firewall enabled (xp with sp2 turns this on by default, for example). Turn off the firewall on one machine and retest your pings. Once you get connectivity in both directions then and only then would I start to look at CA issues.
2nd check, are the pc and the 400 on the same tcp/ip subnet? If not, check your routing tables on both machines and on router in between them.