This is a virus detection. Viruses are programs that self-replicate recursively, meaning that infected systems spread the virus to other systems, which then propagate the virus further. While many viruses contain a destructive payload, it's quite common for viruses to do nothing more than spread from one system to another.
Indication of Infection
Excel's macro warning displayed. Message boxes mentioned above.
Methods of Infection
Opening an infected spreadsheet will infect Excel's startup directory, and then infect other spreadsheets.
EXCEL.97.Bdoc2.A (VirusBuster), Macro.Excel97.BDoc2 (Kaspersky), MX97:Anis family (Alwil), X97M.Anis (NAV), X97M.Bdoc2 (Ikarus), X97M.Bdoc2.A (Softwin, MkS) , X97M/BDoc2.A (GeCAD), X97M_BDOC2.A (Trend), XM97/Bdoc2-A (Sophos)