Quite often the log files are found in the folder you were in when you started the alfresco service. I'm not sure if that's the same in Windows but I presume it is.
Otherwise, most logging is also sent to the tomcat logfile which you should find in <INSTALL DIR>/tomcat/logs/catalina.log - the folder name is correct but the filename itself /may/ be different, check which files you have there which have been modified recently.
