SquidNT + Windows Seven + NTLM

By default, NTLM-authentication is disabled on Windows 7.
To turn it on, open local security (secpol.msc), then Local Policies - Security Options - Network security: LAN Manager authentication level, set it to: "Send LM & NTLM..."

If your Windows 7 system is Windows 7 Home Premium, you may need to configure it via Registry.
1. Launch regedit from Start Search box.
2. Find the following branch.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa

3. Create a DWORD key under Lsa and set:
Name: LmCompatibilityLevel
Value: 1
4. Restart.