The first macro virus to infect Microsoft Excel files (.XLS workbooks), Laroux appeared in mid 1996.
It is a simple virus, similar to the first Word macro viruses, and contains two macros, auto_open and check_files. The auto_open macro is run when the infected document is opened, and merely instructs Excel to call the check_files macro every time a new worksheet is activated.
When this happens, the virus creates a file in the XLSTART directory called PERSONAL.XLS and copies the viral macros into it. This file is automatically opened every time Excel is run, much like Word's NORMAL.DOT. From then on, it infects every workbook used.
When PERSONAL.XLS is infected, the virus will be loaded every time Excel is started.