BBS Server 1.2 by Leif Bloomquist is a modem emulator for Windows.  With this program you can connect a Commodore 64 or any other computer to a Windows PC using a serial cable and the Windows PC will act like a modem.  The special part of the BBS Server is that it can convert a normal Hayes ATDT command into a telnet session letting your old PC connect to BBSes across the internet.  This archive includes the executables, source code and documentation.  Note, there is a newer version of this

Telnet BBS Server is a small Windows program that provides Internet connectivity for Commodore (or other 8-bit) computers. Essentially, it performs these two functions:

Provides incoming Telnet access to 8-bit BBSes.

Provides outgoing Telnet functionality for 8-bit Terminal programs.

BBS Server acts as a modem emulator , appearing as a modem to either end of the connection. This means that the BBS or Terminal program can run without modification. It runs as

BBS Server 1.4a by Leif Bloomquist is a modem emulator for Windows that emulates a Hayes modem.  With this program and a serial cable you can accept incoming telnet connections that your Commodore thinks are coming from a real modem.