I'm running Act 2012 SP1 hotfix 7 on Windows 7 64 bit.
I'm getting the "SQL Database Instance is unavailable" error, but only when I try to open a .wpa attachment that was created in Act 6. The database itself is readable, I can view/change/add records without problems.
I've just upgraded the clients from Windows XP to Windows 7 64 bit. The database is stored on an SBS 2003 machine. The XP machines worked fine before I decommissioned them, and one I've still got left to upgrade also still works fine. The x64 Windows 7 machines give this error trying to open the older attachments.
I've tried a Windows 7 x86 machine, and it also works fine, so I'm assuming some old 16 bit code is necessary to convert the .wpa from the old to new format.