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.
[Analysis: Mikko Hypponen, F-Secure]