I'm running MS Virtual PC on fully patched XP with W2003 as a guest. I
start a ping from guest to host and host to guest, and it will run for
about 10-20 minutes without issue. At some point it just stops/pauses
and I just get "Request timed out" on both sides.  From either side I
can ping another system on the network with no problem - they just
don't ping each other anymore.

Now get this: If I open a browser window to any website on the host,
all of a sudden both environments continue their pings again.  I can
open a browser to an internet site in the guest, but that doesn't
"unclog" the connection between the host and guest.

I don't see anything in event logs, nothing weird in pfirewall.log
which I turned on just for this.  I've temporarily turned off Windows
Firewall for both environments, though I don't like that state.  I'm
running Norton Antivirus 2006 but that doesn't monitor network
traffic.  I'm not running any other a/v, port scanning, or firewalls
except an external hardware firewall.

Can someone lead me to any diagnostics I can use to figure out why the
comms are dropping here?

Thanks.