Considering there will be no cost other than the time downloading and burning on CD a few different versions of Linux, what do you have to loose?
If you can boot off the CD, you can run Linux in the Live mode without installing it and see if it supports your modem, etc.
There is also a many Linux sites where you can get some info that will help.
The XFCE version of Linux Mint offers multimedia support and is based on Ubuntu Gusty Gibbon Version 7.10.
Another option would be Puppy Linux that is very fast on older computers.
Give a few different versions a try.
Do you need to recover anything on the computer before you install Linux on it?