Modem IRQ ?

The Motorola SM56 Speakerphone modem in XP is on COM 3, IRQ 5. It
works fine.

But not in Vista BETA2, running on the same computer, with the same
hardware but a different harddisk. There it is using IRQ 23. No
changes have been made to the BIOS.

In device manager, under the modem's RESOURCES tab, the "change" button
is grayed out. I see nothing in the registry relating to IRQ.

Is there a way to shift this modem to IRQ 5?

Thanks