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.
Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.
Description Created: Mikko Hypponen, F-Secure