FileZilla Server now uses UTF-8 encoding for filenames as specified in RFC 2640 (http://www.faqs.org/rfcs/rfc2640.html). This allows for proper handling of filenames containing non-English characters, as FTP originally was only designed for 7-bit US-ASCII.
In order to support UTF-8, FileZilla Server now requires at least Windows 2000 or higher, Windows 98 is no longer supported.
You might have problems with some non-UTF8 capable clients that do now support RFC 2640. In this case I recommend to use FileZilla as client, it supports UTF-8.