When you inspect your DNS event log, it contains either or both of the following warnings:
<b>Event 5509  (Warning) DNS_EVENT_BAD_UPDATE_PACKET
DNS Server encountered invalid UPDATE message from %1. Packet was rejected.

Event 5509 (Warning)
The DNS server encountered an invalid DNS update message from "IP". The packet was rejected.</b>
You are running Windows 9x clients that have invalid characters in their name(s).

A Windows 2000 DNS server accepts names with a dash (-) and with an underscore _.

All other special characters are considered invalid. When a name registration from one of these computers is attempted, the DNS server rejects it, and issues the 5509 warning.

To resolve the issue, I recommend that you rename these W9x computers.

NOTE: You could (I wouldn't):

1. Start the DNS snap-in.

2. Right-click the server and press Properties.

3. Select the Advanced tab.

4. Toggle Name checking: to All names.