Some doubts about NT and win2k

1.I wanted to know the difference between system files and boot files?

2. When i install winnt i get this message ” Per server for … concurrent connections”

what do i do?

3. What is Exchange Server in winnt?

4. In NT ver4
a) If PDC fails, can we promote BDC to PDC

5. What happens in win2k if the server fails?