Today I received a call from one of my friends who was trying to map network drive in Vista Business (with no Service Pack 1). His account was not administrator’s account and he was not able to see mapped network drive in Explorer though it applied through the Domain Group Policy.

The issue is that by default, Windows Vista uses NTLMV2 for authentication when attempting to map network drives. To allow NTLMv1 or LM challenge-response operations do the following:

1. Control Panel -> System Maintenance -> Administrative Tools (run as administrator) -> Local Security Policies -> Local Policies -> Security Options

2. Find the Policy Key named Network Security : LAN Manager Authentication Level

3. Set the value to "Send LM and NTLM responses" or - and it seems to make the most sense -"Send LM & NTLM - use NTLMv2 session security if negotiated"

This also can be done by making a change in the Windows Vista registry

1. Run the registry editor and open this key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa

2. If it doesn't already exist, create a DWORD value named LmCompatibilityLevel

3. Set the value to 1

4. Reboot

Source: http://youradmin.ca/