F-Secure anti-virus products don't detect any virus by this name. However, the FS 'virus' is a special marker built in to VIRSTOP. Basically, if VIRSTOP reports FS virus, it means that VIRSTOP is not working correctly. Contact F-Secure anti-virus support for more information.
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.