DEL is an internal command of CMD.EXE so it might be neccessary to load it first (ie the line might be something like this: CMD.EXE /C del C:\RECYCLED\?*.*)
Also, command interpreters usually ask for confirmation in case all files are to be deleted (which is exactly what is done by the "*.*" part). That is why I included the question mark in front of the stars. It *does* fool COMMAND.COM from Win98SE, but I'm not sure that newer versions are also that stupid. It does fail by 4DOS (but there are other methods to skip the questioning under that interpreter).