This was a painful exercise. The internet to Asus' website is awfully slow, and
it's not the most friendly site to navigate.
And no, I found nothing like what you describe. And how does it load without
Windows? Do you install it onto a boot floppy or something?
Asus will not provide any diagnostics. Find diagnostics for and
from the Ethernet chip manufacturer.
Long before drivers are loaded and when the Windows splash screen is
apparent, the Ethernet chip is being setup. This may be when the
suspect computer is sending something to interfere with ADSL.
Normally nothing in data should cause that ADSL shutoff (see near
last paragraph for more details). But hardware (electrical) currents
might. Ethernet is supposed to have galvanic isolation to make such
electrical currents not possible. Is it comrpomised? To better
understand what is ongoing, we need something that will reproduce that
problem to know what is (could) produce your ADSL shutoff. A worst
case data test for that Ethernet chip would do that.
Diagnostics are so powerful, in part, because they run as all
computers once executed - without any operating system (Windows). The
trick and power of diagnostics is to execute a known good and specific
software without the complications from OS (such as Windows). But only
the better computer manufacturers provide diagnostics. Normally that
would outrage computer experts. But today, so many computer
assemblers don't know basic diagnostic principles. Instead they only
shotgun - keep replacing parts until something works. Too often,
computer assemblers fix problems using the same joke in Home
Improvement - "more power" rather than fix the problem.
In your case, a failure must be seen before it can be solved. What
is (ie identical computers and identical setups) does not exist until
confirmed. Whatever your problem is, the problem is indeed subtle.
To understand it, for example, I would have put an oscilloscope on
that Ethernet port to see what is happening - another diagnostic
tool. But you (and most every computer technician) would not have
that tool nor the experience to immediately see it on the scope.
Hopefully the Ethernet diagnostic echoing data between computers
might make the problem so reproducible or apparent as to then be
identified.
Everything you have done implies no problem generated by hardware.
But your problem is so typical of a hardware anomaly.
Unfortunately, a third 'similar' computer is not available. Another
test would execute that diagnostic data test between two 'good'
computers. Then power on the suspect machine. Or setup computers in
Windows to do larger and repeated file transfers.
It is possible during startup that the suspect computer sends a
collision signal. This collision signal would temporarily stop
diagnostic data. However this signal should not affect ADSL on
another side of the router. Should not. Just another reason why
better hardware provides diagnostics.