"I want to change my windows 2003 server with Linux. What step by step procedure i must do ?"
.. jahh that'S a good one .. .. if you start this .. then you'r in charge of everything to choose ..... what / how you want to use it .. everyone likes it different ...... also depending on which features of 2003server u use.
but to get something like 2003 you could try to implement:
-> do you mean PDC/BDC ? ( Domain Controller NT-times)
-- isc-dhcpd -> http://www.isc.org/index.pl?/sw/dhcp/
-- bind as your DNS http://www.isc.org/index.pl?/sw/bind/index.php
-- samba as your fileserver http://us1.samba.org/samba/
-- LDAP / MYSQL for centralized authenticating of users
2. Mail Server.
- you could use exim -> http://silverwraith.com/vexim/pop3imap.php
3. Proxy Server.
4. 'Exchange server".
-> what features of exchange do you use so exzessively that it's an extra point ?
there is a gpl alternative out there for everything ....
one thing for the way: using linux you first have to understand the concept .. then your set ... .. im also still trying ... but getting better