This is from a document I've had for years. Not sure where it came from but if you follow it then it works.
Installing and using CoSession on Windows 9x / NT 4.0 / 2000 / XP
Computer
The computer used as the CoSession client must be an x86-based PC desktop or notebook system meeting the minimum requirements for the chosen operating system.
Modem / Serial cable
An AT-command set modem is required for remote connections via modem. A Null-Modem cable is required for use in a Direct-Connection. Additionally:
• When using CoSession on Windows NT 4.0, Windows 2000, or Windows XP, COM ports can be initialized for use at up to 57,600 bps. Using those operating systems,COM ports cannot be initialized for use at 115,200 bps.
• CoSession file transfer operations perform reliably up to 38,400 bps.
Installation Instructions
The following instructions are recommended for all operating systems and are required for Windows NT 4.0, 2000, and XP.
1. Insert the CoSession 7.0 Compact Disc in the CD-ROM drive.
2. Go to the CD-ROM drive directory and double click “Setup.exe”
3. Supply your name and company’s name for the dialog and click “OK.”
4. Click “OK” to accept the default installation directory: C:\Insync\Csw
5. When prompted for install options, choose “Custom Install“ and then click “OK.”
6. Clear the “CoSession for Windows Host” selection
7. Confirm that “CoSession for Windows Remote” is selected, and then click “Install.”
8. Clear the "NetWare SPK," "NetBIOS," and "WinSock" selections.
9. Confirm that only “Modem/Serial” is selected, and then click “OK.”
10. Continue by following the on-screen instructions.
Simply put, it MUST be Custom, Host Only, Serial Only. Selecting any other settings may render the installation useless.