The COM port is built into the motherboard and the BIOS controls its IRQ, so I can't change it, at least not easily in any way I can see.

I don't want to change the modem's IRQ at the moment, as that involves physically opening the case. I understand this would be an option, but I'm trying to avoid that at the moment because this is clearly a software problem, not a hardware problem. I don't like the idea of having to resort to hardware changes that I shouldn't need to make.