This is not a virus. Actually, the typical support query goes like this:
Question: I found a file called "nul" in my root directory. I can not access or delete it. Later I found it existed in each and every directory! This has to be a virus!
Answer: How did you find it? With which program?
Try locating in a same manner files with names such as "lpt1", "prn", "com1", "clock$" and you will see that they too exist in every directory and can not be accessed or deleted. This is normal and is a feature of DOS.
These are virtual names for system devices: prn for printer, nul for null port, clock$ for CMOS memory etc. Ignore them.
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.