XP SP3 does improve the speed of kernel dispatches - by about 10%. This means that calls to system services take a wee bit less time. This probably isn't accountable for much of the speed increase you're seeing, but it does factor into the mix.

The command  "regsvr32 hnetcfg.dll" re-registers the WMI DLL with the OS, and that could improve the wall-clock speed of browsing quite a bit over a DLL that has to be re-loaded for every call through the API.

And I've read that Norton 2009 sucks less than the prior versions.