Router network: XP PC able to see Xubuntu PC, Xubuntu does not see XP PC !

[Browser_ice]

I have a Dlink router.
One XP PC connected to it. IP=192.168.0.101
One Xubuntu PC connected to it. IP=192.168.0.100
Both PC access Internet.
My XP PC sees my Xubuntu PC. Pinging works.
My Xubuntu PC does not see/reach my XP PC. Pinging does not work, unreachable.

I have no filters at all, default configuration.
I have no sharing setup.
I have no firewall.

How can I have both PC be able to see each other ?

[Browser_ice]

I guess its a question too hard for you guys.