Cannot join a Windows NT domain with a Windows Vista Business SP1 PC

I am trying to test if Windows Vista can be used for our current software at work as there is very little choice when purchasing new PC's and laptops these days. I have already failed the first test!
When I try to join our company Windows NT domain I get the error:
"The following error occured attempting to join the domain 'domainname'.
Logon failure: unknown user name or bad password"

I have done the following based on searches:

1. Added computer name to NT domain via NT Server Manager
2. Made the following changes to the Vista PC's Local Security Policy:
Local Security Policy (start | secpol.msc) | Local Policies | Security Options
Domain member: Digitally encrypt or sign secure channel data (always) - Disabled
Network security: LAN Manager authentication level - Send LM & NTLM - use NTLMv2 session security if negotiated
3. Disabled Windows Firewall
4. Checked that the PC has a WINS server address and it is correct
5. Some have mentioned a blank MAC address but MAC address looks ok.

I have run out of ideas.

And no, I cannot upgrade the domain controllers to Windows 2000/2003 or anything else.

Please help. Others seem to have worked this one out and I feel a bit sheepish!
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With the default settings Vista won't authenticate to a NT domain. You have to lower the authentication level Vista uses. "Run as administrator" secpol.msc Under Local Policies > Security Options Change the value of "Network Security: LAN Manager authentication level" from "NTVLM2 responses only" to "LM and NTLM - use NTLMv2 session security if negotiated"
if this dont work then you need to upgrade your domain to win2k or win2k3
The Security downgrade looks fine to me. is this VISTA RTM?

Has the Vista Machine got IPv6 Enabled? if so try disable it and then try again.

Also try using the wizard, Right click on My Computer > Properties >
Computer Name > Network ID.

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DGIUKAuthor Commented:
I think using the wizard did it. I had already disabled IPv6 cannot be sure if it was this or using the wizard instead of clicking "Change" and doing it manually.
Either way I now have a Vista PC on our domain. Although thats probably not a good thing!
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