To allow Windows Vista clients to join a PDC that's hosted on Mac OS X Server v10.5 or later, the local security policy of the Windows Vista client must be changed. On each Windows Vista client:
- Choose Start > Control Panel.
- Select "Classic View".
- Open "Administrative Tools".
- Click the Shortcut named "Local Security Policy" and confirm that you want to continue.
- Select "Local Policies" and then select "Security Options".
- Scroll down the list until you find the "Network security: LAN Manager authentication level" policy.
- Double-click "Network security: LAN Manager authentication level" to edit this policy.
- The default Windows Vista setting is "NTVLM2 responses only". Change it to "Send LM & NTLM - use NTLMv2 session security if negotiated".
- Click the "Apply" button.
You should now be able to log in.
For an explanation of each of the various options for this security policy in Windows Vista, see this Microsoft Technet article.