That is a nasty bug in Tiger, and the only workaround I know of is to lock your network settings.
We had a long discussion about this on MacFixitForums, while it was still reachable. I can't find that thread anymore, but I posted of the gist of it to MacOSXHints, in the
Dialog From Hell: Your network settings have been changed by another app thread.
As explained there, there's actually a pretty simple permanent fix.
As for the network USB adapter triggering the problem, my guess is: as part of the installation, you needed to type in a password for the new network interface. Sometime later, you installed the Apple Security Update that tries to harden the protection for that password, and gets all discombobulated trying. If you want to point fingers, point at Apple. It's their Security Update that dropped the ball.
Or better yet, if you just want to fix the problem:
a) Lock the padlock in System Preferences (anywhere you see it, for example in the Security pane)
before going to the Network pane. That's just to temporarily hush the dialog.
b) Go to System Preferences -> Network. This is safe as long as the padlock is already locked when you arrive on this pane.
c) Now you can unlock the padlock again.
d) Run through all of your network settings. For every location, for every interface of that location, glance at the PPP or PPPoE or PPPoA subpane. You're looking for anywhere you have "Save password" checked.
e) Uncheck "Save password". If you want, you can turn it right back on by re-entering the password. The important thing is that the password be entered
after applying the problematic Security Patch.
f) That's all. Problem fixed. If you want to check that you didn't miss any passwords, leave the padlock unlocked, leave the Network pane, and come back. If you
do not get the "Your network settings..." error message, you're completely and permanently healed.
The script that virtual1 developed automates the above for you, without forcing you to go through the GUI. Last time I saw it, it did not re-enter the password for you, although we discussed several ways to recover it if you've forgotten, and those may well be in the current version of the script. (There's a reason for that Security Update; it was just too darned easy to retrieve passwords that should have been encrypted better.)