X97M/Diablo is an Excel 97 macro virus.
Disinfection & Removal
When a sheet is activated from an infected workbook, X97M/Diablo.A if a workbook "PERSONAL.XLS" is exists in the Excel's startup directory. If the workbook does not exist, the virus creates a new workbook and infects it. After that it infects all workbooks that are opened.
The virus activates its payload in the random dates, when it changes the size of toolbar buttons to large.
Technical Details: Katrin Tocheva and Sami Rautiainen, F-Secure