When an infected workbook is opened, X97M/Manalo.A checks if file
"MANALO.XLS" exists in the Excel's startup directory. If the file does
not exist, the virus creates a new workbook, infects it and saves it
to there.
At October 3rd, 1999 and every day after that the virus activates its
payload. At this time it shows a hidden sheet with some text and a
flag.