OK, try something like this...
setserial -v /dev/ttyS0 uart 16550A port 0xcff0 irq 4
(or irq 5, which also seems to be unused, and would help avoid needing
to mess with the BIOS setup since your "normal" serial ports won't be
after it.)
I think it's the lack of the "port 0xcff0" which is causing the
"input/output error"; it'll be looking at the standard port, 0x3f8.
What you set in serial.conf should follow what the BIOS setup has been
configured to; ie. you configure the BIOS setup, and then change
serial.conf to correspond with it (if the kernel hasn't figured it out
automatically).
And, your modem really is on ttyS0 (COM1)? Do you have a serial mouse?
Not sure what type. It's a Logitech PS2 with /dev/mouse pointing to
/dev/psaux.
OK, that's a PS/2 mouse, not a serial mouse. I'm just trying to figure
out what's using your serial ports. Looks like nothing is, since it's
a PS/2 mouse and an internal modem. So it's safe to disable your
serial ports completely in the BIOS setup.
Well, it may not be necessary since I've now realised that your modem
is an internal modem with a genuine UART, not an external one. Because
it's not part of the motherboard, it won't have any settings in the
BIOS. But since it seems that you're not using them, it wouldn't hurt
to turn off your motherboard's serial ports in the BIOS, to make sure
they're not after any IRQs.
If tweaking the modem's IRQ doesn't work, it might be possible to
tweak the ethernet card's IRQ, by loading the module with something like
modprobe 3c59x compaq_irq=5
I'm not too sure about this, though; the kernel docs explain the
"compaq" bit as being a workaround for a Compaq bios problem, but
don't say what that problem is. I'm not sure if this would actually
change the IRQ on other machines. My guess is it probably would, but
I'm not sure. (Or do you have a Compaq?)