To get Windows 7 clients to connect to the domain running Samba 3.3.4 or higher, four registry keys need to be changed. For the ones that don't exist, create them.

Two dword keys within HKEY_LOCAL_MACHINESYSTEMCurrentControlSetservicesLanmanWorkstationParameters:

"DomainCompatibilityMode" = 1
"DNSNameResolutionRequired" = 0

And two within HKEY_LOCAL_MACHINESYSTEMCurrentControlSetservicesNetlogonParameters:

"RequireSignOnSeal" = 0
"RequireStrongKey" = 0