[UFO Chicago] Central Authentication; Replacement for Windows

[Douglas Thompson]

I'm looking for the best and most secure way to replace an existing
Windows domain infrastructure with a Linux infrastructure.  The Linux
solution should provide central authentication, email and global
address book.  This new environment will be serving Windows based
clients, and should, if possible, implement a single sign-on
mechanism, whereby, allowing all authentication tokens to be updated
by changing one password.  I've looked at qmail, samba and openLDAP,
but am not sure if this is the correct method or if it will support
all requirements.  Any ideas?
Thanks,

Doug