It seems that Filezilla, my FTP client of choice for Windows, doesn't go too well together with Service Pack 2, more specifically with the built-in firewall. It does work, but the initiation of transfers is irritatingly slow, sometimes taking up to secons for a single transfer to start. And queuing the transfers doesn't help much.

It seems that having a firewall rule set up in the XP firewall settings specifically for Filezilla (to allow any traffic to and from anywhere on the network) doesn't help either. So as of now, I've disabled the thus far otherwise non-problematic built-in firewall, since most of my use of Windows does involve heavy use of FTP, and it's easier this way than changing the app.

Currently this problem goes unmentioned in the Filezilla FAQ, although there is a generic warning against using any personal firewall software since they're known to be the most typical culprit of trouble with this sort of applications.