Hmm. I'm not sure why you are encountering such a problem with so few internet applications. Normally you run out of buffer space when using 100s of sockets at once or If a program is poorly coded the programmer may negect to free the socket causing a resource leak. Sooner or later all sockets will be exhasted and a reboot will be required.
Of course FlashFXP doesn't have any such leaks.
Version 1.4.3 (build 835) is not 100% Windows XP compatible and there are several major issues that were resolved in v2.0. So you may want to try v2.0 RC3, you can download it from
http://www.flashfxp.com/rc.php