Open a command prompt and use these commands to first rename and then delete the NUL file:
C:\> rename \\.\C:\..\NUL. deletefile.txt
C:\> del deletefile.txt
Using the \\.\ prefix tells the high-level file I/O functions to pass the filename unparsed to the device driver – this way you can access otherwise invalid names.
Read this article about valid file / path names in Windows and the various reserved names.