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  Note, there is a newer version of this software (1.4a at the time of this writing) that you should probably use, but I'm adding this to the collection for completeness sake. 

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 a background task and uses negligible CPU time.

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.