This is a workaround for making Windows 7+ play nicer with XP dominant environments.
Navigate to: Administrative Tools > Local Security Policy > Local Policies > Security Options
Change the following options:
Network security:Minimum session security for NTLM SSP (including RPC based) Clients Properties
Network security:Minimum session security for NTLM SSP (including RPC based) Servers Properties
Network Security LAN Manager authentication level Properties