reply to how to join windows clients.
first of all let me answer your second question.
open a terminal. Login as root. issue a command
select Use LDAP
select use LDAP Authentication
select Use TLS
provide your IP of active directory server.
Provide domain details.
Thats it. Your Linux Machine is joined to the ADS.
Installing Nagios is pretty easy. just unpack the tar.gz archive and read the Documents carefully. Each and every step is clearly mentiones here.
About Joining a XP client to Samba PDC is somewhat tricky, Even I did not get the right solution for that. I will revert back as I get the solution for that.