External fax modem fails to initialise after XP SP2 installed

I have an external fax modem connected on COM1 working ok prior to SP2
being installed.

In particular, I was able to leave the modem powered off most of the
time, and could power on the modem just prior to sending or receiving
a fax.

If I powered the modem on after XP started, I would see the lights
blink and TR would light and remain lit.

With SP2 installed, powering the modem on after XP starts results in a
blank display. TR lights for a short time, then extinguishes.

If I reboot the PC, and leave the modem powered on, XP notices and
initialises the modem soon after I log in, and the TR light remains
lit.

Can anyone offer any suggestions?

How might I debug this problem?

Earl