When you inspect your DNS event log, it contains either or both of the following warnings:
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.
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.