First off, for 99% of us in the universe nowadays,
C64Term for the PC
 (as suggested on the website) simply won't work.  Pretty much every
 computer sold in the last 10 years is equipped with a Winmodem, and
 real "hardware" modems have been impossible to find for the last 5. 
Of all the places I would hope people wouldn't be stuck with only the
latest Wintel trash (and yes, I consider a computers such as you
describe "trash")...
...but I suppose we can't all be that lucky.
  I personally don't know of a better solution,
other than to use a C64
 emulator and bind to whatever modem people have.  I don't know what
 the answer is, but hopefully somebody can figure out a
 Windows-compatible solution. 
VMware running a real OS, presenting the winmodem as an ordinary serial
port (of whatever type it deos for serial ports)?
I know someone who does that for, IIRC, video - his laptop's video (or
whatever it is) isn't understood by his OS of choice (a Unix variant,
don't recall which one), so he runs Windows and VMware, with *nothing*
else running directly under Windows; everything "real" runs on the
VMware emulated machine.
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