mysterious loss of internet connection

[Steve Kleene <skdeb@syrano.acb.uc.edu>]

My Etch machine at home is connected to a DSL service via a Netgear hub
(router) at home.  Sometime in the last two days, the machine lost its
network connection to the outside world.  It seems to be a software problem.
It had been fine since I built it in January.

Here's what I know:

1. If I boot from the XP partition, I have full internet service anywhere I
   try.  So the hardware seems OK.  There is only one NIC.
2. Running Etch, I can ping the router (192.168.0.1) and check its software
   configuration in iceweasel.  The router configuration is the same as it's
   always been.  It shows that the router has received an IP address
   (10.something) from the ISP.
3. I cannot ping or otherwise reach any site past the router by name or by IP
   address.  If I try, it times out.
4. Rebooting doesn't help.
5. The output of "ifconfig -a" looks normal.  It shows eth0 on 192.168.0.4,
   UP BROADCAST RUNNING MULTICAST.  I can post the whole output if necessary.
6. I haven't done any upgrades with apt since five days ago, which is clearly
   before the problem started.
7. Nothing jumps out at me from dmesg or /var/log/messages.
8. I found the iceweasel cache files from my last successful browsing.  Then
   I used find to list every file on the system that had been modified since
   then.  Again, I didn't recognize anything scary.

I'd appreciate any suggestions on how to continue chasing this down.  It may
go slowly, since the problem is at home but my only connection now is at
work.

Thanks.