This is a family of at least 11 small, non-interesting viruses, which add their code in front of any files they infect.
Depending on the settings of your F-Secure security product, it will either automatically delete, quarantine or rename the suspect file, or ask you for a desired action.
More information on the scanning and removal options available in your F-Secure product can be found in the Help Center.
You may also refer to the Knowledge Base on the F-Secure Community site for more information.
An extremely badly written variant, which crashes on many machines.
This 301 byte variant activates in September and displays the text:
Sad virus - 24/8/91