If you set up the Linux box to act as a domain
controller (using Samba), the individual _users_ of
the Windows boxes will be authenticated. My real
question to you is what do you mean by "log in"? If
you mean "log in" as in use the Linux machine's apps
and such, you would need to do that thru LTSP and
forget about Windows. If you mean "log in" in the
manner of Windows Terminal Services, you'll have to do
some investigating. I don't believe
LTSP(http://www.ltsp.org) handles that kind of
functionality. Personal Note - Windows doesn't seem
to handle that kind fo functionality either but it
still tries.

Here is the existing situation explained:

I have 4 DOS PCs and 2 Dumb Terminals which I have connected to an Intel 486 machine running AT&T Unix SVR 4.2. The two Dumb Terminals are connected to the serial ports (RS 232) of the Unix machine. The Unix machine also has a 4 port (RJ-45) terminal card. The 4 DOS PCs are connected thru serial ports to the 4 port terminal card of the Unix machine. The serial ports and the 4 port terminal card of the 486 Unix system are configured in Unix through port monitor and port service utilities of Unix. I directly get the login prompt of the Unix system on the Dumb Terminals. While in the 4 DOS PCs I run a terminal emulation software (beeline.exe) that helps me get the login prompt of the Unix system. I can now log into the Unix system from the 2 Dumb Terminals as well as the 4 DOS PCs, now acting as terminals.

Here is the desired situation explained:

I would like to replace the Intel 486 Unix system with an Intel Pentium P-4 system running Linux (Red Hat 8.0). How can I retain the connectivity with the Linux machine that I had with the 486 Unix system as explained above?