Do these adjustments on the Windows 7 machines that need to connect to the server. I'm not sure if this has to do with your problem but this adjustment is usually required to allow Windows 7 to work with older servers.
Control Panel - Administrative Tools - Local Security Policy
Local Policies - Security Options
Network security: LAN Manager authentication level
Set to Send LM & NTLM responses only
Set the Minimum session security for NTLM SSP
Disable Require 128-bit encryption
The screen shots below should be helpful.
Reboot all machines after making the adjustment.