How to disable NetBios Null Sessions

Below are instructions on how to disable NetBios Null Sessions:

Windows XP

Go to Administrative Tools --> Local Security Settings --> Local Policies --> Security Options. Make sure the following two policies are enabled:

Network Access: Do not allow anonymous enumeration of SAM accounts - Enabled

Network Access: Do not allow anonymous enumeration of SAM accounts and shares - Enabled

Windows 2000

Go to --> Administrative Tools --> Local Security Settings --> Local Policies --> Security Options

Select "Additional restrictions of anonymous connections" in the Policy pane on the right

From the pull down menu labeled "Local policy setting", select
"No access without explicit anonymous permissions"

Click OK

Windows NT

Set the Following Registry Key:

HKLM/System/CurrentControlSet/Control/LSA/RestrictAnonymous=1

Borrowed from brown.edu