Can ping, cannot connect

[HLAMUTHNOSPAM@EARTHLINK.NET]

I have a three-computer home wireless N network. It was working like
a charm, and then it wasn't. All computers use Windows XP home Sp3.
All computers can connect to the named master, but the master cannot
connect anymore to one of the other computers. It can ping it but not
access it. All permissions are set right. I use McAfee fire wall and
network manager. Nothing seems mis-set. I am stalled. Reboots don't
help.

All suggestions welcome.

Henry