What is the purpose of:
C:\WINDOWS\system32\config\Software.log
How to open it.
The file contains the journal of the HKEY_LOCAL_MACHINE\Software hive. If the machine crashes while in the middle of updating a registry key in that hive, the .log file is used to recover the registry. I would recommend not messing with the file unless you know what you are doing.
It is part of your Registry, you cannot open it directly