Every so often, somebody reports that whenever they open a command prompt, they get a weird error message between the version banner and the prompt.

Where is that message “The filename, directory name, or volume label syntax is incorrect” coming from?

My psychic powers tell me that it’s coming from AutoRun.

AutoRun is a hidden gotcha of the command processor which lets you set a registry key to inject a command into every command prompt as soon as it starts up. And I’m guessing that there’s a rogue AutoRun entry that is doing something which is generating that message.

For example, I was able to reproduce the message by doing this:

reg add "HKCU\Software\Microsoft\Command Processor" /v AutoRun /d \"C:|

After running this command, open a command prompt, and you’ll get that same error message.

And before you forget, run this command to get rid of the bogus AutoRun:

reg delete "HKCU\Software\Microsoft\Command Processor" /v AutoRun /f