A registry change at login or a system policy can turn this on at every logon. Tips Downloads from the Internet should be restricted or postponed in the critical Y2K period. At least a communication should be made to the Internet users to make them aware of the potential dangers. Don't open attachments you receive by email unless you know exactly what the document is. Even if the sender is somebody known, you must pay attention.
Viruses like to hide in an unsuspected environment.
A macro virus can only be harmful if it is allowed to run, so disabling the macros allows you to open the workbook safely. Backups won't prevent viruses. But if one ever hits you, the value of backups is immeasurable. Keep your PC clean. Well-known ways of infections are emails and diskettes from PC's.