Knowing that WS FTP is a client software, what exactly do you mean by "backing up sites"? Whose sites are you trying to "back up" and are they Ok with it?

If you have a site outhere and are trying to backup its content to your local computer, say, every night, then you do not need any client software like WS FTP to do that.
All you need is to use scheduler to schedule a run of a batch file every night at a certain time. Lets name this "myFTPbatch.bat". The batch file would look somewhat like this:
cd \ftpbackup
ftp -n -v -i -s:ftpcmds.ftp
the ftpcmds.ftp would look similar to this:
open 0.0.0.0 (the necessary ip address or a name address of the ftp server to connect to)
user johndoe password (after the "user" enter the appropriate user and password)
cd pub (if you need to switch to a certain directory on the ftp server, otherwise disregard)
binary
mget *.*
bye
notes:
make sure that you have ftpbackup folder in root of the drive and ftpcmds.ftp file resides there, the same directory where you want the files from the remote server to be placed.
Good luck