The regsvr32 command doesn't actually delete any file(s), it just unregisters the file from the system (Windows, in the registry) so that  any program trying to use it won't know where to find it. But to answer the question about the command having trouble actually finding the DLL, be sure to put the entire command in quotes if the files path contains any spaces, (which the Program Files folder does). So it should look something like this when you type it in the DOS window:

"C:\program files\file folder\filename.dll"

Geek Info:
The reason it needs quotes is because DOS is an old 16-bit operating system and only understands short filenames and folder paths (8 characters long, a dot, and then 3 characters long. i.e. MyFilenm.exe). Windows is 32-bit or 64-bit and understands long filenames and paths (i.e. My File Name Can Be This Lonnnng and have spaces.exe), so to "force" DOS to understand long filenames/paths you just wrap it in quotes. (More info here if you're interested)

what are its file attributes?

What is the reason you are trying to delete a .dll file out of your program files dir?  As stated regsvr commands are only for registering and unregistering dll's.  when you brows to the file location do you actually see that dll? If you do a search can you find that actual dll.  Maybe your path is incorrect when your typeing your command or as dogg stated you need to include the quotes.  At any rate if you can see the dll file try to simply delete it. If it won't let you delete it you could try doing it through safe mode or even simply try renaming the file.  Be careful though that you really want to be removing a dll file. It may be used by other applications.