Hope this helps.
The log file is wherever you tell it to be
Default location is noted above.
You can search for it by opening a command prompt, changing to the root of the drive, and running the command: dir ondemandscanlog.txt /s
If it doesn't exist, you can examine your VSE configuration (if you're in a standalone environment) or your ePO policy (if you're in a managed environment) to verify that logging is enabled and where it is configured to write the log.
Thanks guys the joys of windows 7
As a helpful addition, there is a SET variable DEFLOGDIR created by the installer, so you can always refer to the default log location by using %DEFLOGDIR% in references (such as paths).