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