400 Directories! Who do you work for, the Post Office?
In Windows ME, Alter F, Enter, Enter in Explorer will create a new folder with the focus ready to type the new folder's name. That would work fine for a few dozen new folders, but I wouldn't like to do 400.
A keyboard macro program could be used with replaceable parameters. Are the names going to be sequential?
Just a few pointers. I hope this helps.