How to Use Notepad to Create a Log File

To create a log file in Notepad:
1.Click Start, point to Programs, point to Accessories, and then click Notepad.
2.Type .LOG on the first line, and then press ENTER to move to the next line.
3.On the File menu, click Save As, type a descriptive name for your file in the File name box, and then click OK. When you next open the file, note that the date and time have been appended to the end of the log, immediately preceding the place where new text can be added. You can use this functionality to automatically add the current date and time to each log entry.

Source