Prompting for Credentials for Server 2008 File Shares

I have a Windows Server 2008 domain.  I have a Windows Server 2008 machine sharing files to users in the domain.

If I go off network, to my home PC for example (Vista), VPN into our company network, and attempt to connect to these file shares using fully qualified paths, I am not prompted for user name or password (credentials).  I can see the shared folders but attempting to open them gives me access denied.

This behavior is also occurring with my user's home XP machines.

What setting(s) do I need to change to make any non-domain machine be prompted for credentials?  And are all the setting(s) I need server side setting(s)?
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What permissions do you have on the shares/folder?
propracticeAuthor Commented:
Lets take an example folder named Information.  It is shared as \\Division\Information

The share permissions are
IT (PROPRACTICE\IT) - Full Control
ProPractice Users (PROPRACTICE\ProPractice Users) - Full Control
Domain Admins (PROPRACTICE\Domain Admins) - Full Control
Administrators (PROPRACTICE\Administrators) - Full Control

We just transferred from Server 2003, and these things worked.  We did not upgrade the server install, we installed anew.  I have previously removed Everyone because that allowed unfettered access to the outside world.
Try disabling UAC on 2008 box
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propracticeAuthor Commented:
Still does not ask for me user name and password when I \\networksharename to it over VPN.
propracticeAuthor Commented:
We can leave the VPN out of this; mine is one of multiple domains at this institution, and it doesn't prompt me for UN/PW if I am not part of a domain.  If I am part of another domain here, it does prompt me.
This may get you going, although may not resolve the cause.

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propracticeAuthor Commented:
Disabling the domain guest account did it.
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Windows Server 2008

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