FTP trouble on Windows 2000/XP

We have an old HP machine which we often ftp to in order to retrieve files. Recently, our 2000 and XP machines have been unable to do so, while our other machines still work. The message received is something like “host unreachable”. I can ping the HP machine on both 2000 and XP. Any ideas as to why this problem would pop up out of the blue? I haven’t changed any permissions, etc. on the Windows machines, and the old HP system (I’m talking ancient) is the same as always. Help!