You are trying to overwrite a file that is currently open (in use) or locked by another program either on your machine or over a network. If you are sure none of the files you're overwriting are in use, contact Adobe RoboHelp Support at the link below.
I've been creating these builds for years and this is the first time I've received this error message. There are no files that are currently open or in use by another program that I know of. Like I said, I've done these "Publishes" for numerous iterations and this is the first time I've had this problem. I've already asked Adobe RoboHelp for support, but they no longer support my 8-yr-old version. I put the question out on the Adobe forums, but received only one suggestion, which did not work. It was actually a workaround (publishing to a new folder).
Have you updated any Adobe products lately? If you have, you might try a System Restore back before the update. Adobe may have something running in the background.
If the program is 8 years old, you might also try right clicking the Shortcut or the EXE, select Properties then compatibility and run the program in compatibility mode for an earlier verson of Windows.