Serial ports are kind of outdated, There is an option found easily
though. Try using a serial to USB adapter, You may have trouble
getting your modem recognised but I bet there is a workaround to that.
Ubuntu is downloaded as an iso image (CD disk image) and can be loaded
onto a USB drive. Using another computer you can burn the image onto
the USB drive as dialup is much to slow for a download. Set your
computer bios up to boot from USB device and you should be set.
I am using a WD external USB drive for backup and when using another
computer. I have a bootable sector on it and it is loaded with Ubuntu
8.10 I no longer have to put up with Windoze even when away from my
box. You can do it with flash drive but I don't leave anything home
but my computer, kind of neat.