This is a typical boot sector virus. It sometimes displays a 'Runtime error' message.
Defo was reported to be in the wild in several countries during summer 1996.
Based on the settings of your F-Secure security product, it will either automatically delete, quarantine or rename the detected program or file, or ask you for a desired action.
Find the latest advice in our Community Knowledge Base.
See the manual for your F-Secure product on the Help Center.
Submit a file or URL for further analysis.