DNS: Primary and Secondary

I have a question related to DNS and failover for DNS.

Here is our setup
Domain Controller 1: Windows 2003, hosts just GC and DNS.
Domain Controller 2: Windows 2000, hosts DNS, (no GC), DHCP, and also hosts file and print sharing.
Clients: all clients are Windows XP

Here’s the situation. Our client and servers all use DC1 for the primary DNS server and DC2 for the secondary DNS server.

Why is it that if I restart DC1, computers cannot do DNS lookups? Shouldn’t the client computers fail over to DC2.

It is possible that because a client can successfully Ping DC1 while it is restarting, it looks for a response ONLY from DC1?

Thanks for any help!