When you say "user accounts," that implies more than one. Typical FTP is single-user only, meaning that multiple FTP usernames can't log into another FTP user's home directory. PLC1 can only FTP as PLC1 and connect to /home/PLC1. PLC2 won't be able to FTP as PLC2 and upload to PLC1's directory. And if you created a user like CENTRAL, nobody but CENTRAL can upload to /home/CENTRAL.
You said you were able to use WinXP's FTP program to access the user account. Could you upload as well? If so, then you should be able to use WinXP at the remote sites in the exact same way.