network diagnostics on winxp

i've been having problems with my connection to the
internet...i normally use a wireless home network with
cable modem. lately the connection is erratic, sometimes
it finds the site, sometimes it doesn't. for
troubleshooting, i connected directly to the modem and
the problem still exists. My ISP says every looks fine
on their end. now when i have gone into ipconfig and
run manual pings and tracerts, everything went through,
no packets lost, one address at a time. Today after weeks
of fiddling, i found the network diagnostics tool! happy
joy. so i ran it. and it tells me that 3 of 4 pings
failed because the destination net is unreachable. now,
is that a problem on my end or cound those addresses that
the system scan pings be defunct? and what could be
happening?
and another question...i can change the firewall setting
on my 1394 connection in network control panels, but i
cannot enable or disable icf in my LAN properties. why
is that?

i will be so grateful to any who reply,
Suzanne