Over-ruled priority of modem?

I set my modem to atm2 so that I can diagnose connection problems.
Right now, the problem seems to be that XP/SP2 is giving priority to
my hard-drive over my modem. I know this, because whenever the drive
light comes on, the audio on my modem cuts out to silence, and if that
silence goes on for long enough, then it does a re-train.

The only way for MicroSoft to do this is to use the system timer to
over-ride my programmable interrupt controller. Ever since the
eighties, interrupt controllers gave priority to the system timer over
the keyboard over the modem, over the printer, over the hard-drive.
The only way to over-rule this prioritization is to abuse the system
timer.

That's what XP does. It abuses my system timer, so that I must be
careful of what applications I start when I'm on the internet.