Just a guess, but I think XP SP3 includes some of the networking
enhancements that were released with Vista. I don't think Network Access
Protection would cause problems unless you've enabled the feature, but the
tcp "enhancements" caused lots of network slowdown with older network
drivers or routers/switches that didn't support the new features. Features
include sliding receive windows, chimney offload and a few other
optimizations that apparently work great when everything in the network
supports them, but can fail miserably when support is lacking.
Search for Vista network slowdown issues and maybe some of those fixes would
help. Basically, try installing the latest network drivers for your network
card or motherboard. Drivers should be within the last year or so if
possible. You can find instructions for disabling these new network features
in Vista, and maybe the same instructions work in xp sp3.