/Log (devenv.exe)
Visual Studio 2010
Logs all activity to the log file for troubleshooting. This file appears after you have called devenv /log at least once. By default the log file is:
%APPDATA%\Roaming\Microsoft\VisualStudio\<version>\ActivityLog.xml. However, you may specify an alternate path and file name.
where <version> is the Visual Studio version.
Devenv /log <path/name of log file>