Vista - FTP download problems

Hi, all.

I can download at normal speed using IE7 as an FTP client, but other clients
(FileZilla, SmartFTP, even the DOS client) download at an absolute crawl.
(e.g.: ~200Kbps in IE7, ~1.8Kbps elsewhere.)

Any idea what's likely to be causing this? (It's clearly some form of
security/access restriction, but I've no idea what it could be. I've tried
disabling the firewall but this makes no apparent difference.)

Using Vista Business, with all service packs as of the time of writing.

Thanks in advance.

John Parker.