Computers stopped sharing

I have 3 computers: 1 desktop xp, 1 laptop xp and one desktop win98. I also
have one ethernet connection to the TV. Until about 2 days ago everything
was working ok. I haven't made any changes (deliberately). The desktops are
both connected to the router with ethernet. The Laptop is wireless.
From the desktop xp and the laptop I can ping the win98 and TV ok. From
the win98 I can ping only the tv. I can't ping the desktop xp from either of
the other computers and I can't ping the laptop from either of the other
computers.
When I go to My Network Places on the laptop it shows the laptop and the
win98 shared files and where the desktop xp name should be it shows "Unknown"
and under "Unknown" it shows the shared folders but I can't accesss them.
When I go to My Network Places on the xp desktop I see the xp desktop and
win98 shared folders but I don't see anything for the laptop.
Any ideas what I might have done to cause this and how I can fix this?
Thanks, Marc