When an infected workbook is opened, X97M/Friend.A disables Excel's
built-in macro virus protection. Then it creates a workbook into
Excel's startup directory, "sys.xls", and infects it. During infection
the virus creates a temporary file, "C:\office.sys".
After that has been done and the Excel has been restarted, every
workbook that is closed will be infected.
X97M/Friend.B is a slightly modified variant from X97M/Friend.A.
[Analysis: Sami Rautiainen, F-Secure]