This is what i know....
when i first installed XP pro sp1a onto a new comp. and setup the basics in msconfig, performanc, services etc.... it shut down realy quick, like under 3seconds.
after installing everything, such as... win updates, .net fram work, nod32 antivirus, spybot S&D, etc etc.
i had nothing setup to run activly except nod32, so there were still about under 18 proccess running... but after everything was installed it now took 10 to 15seconds at shutdown... So im pritty shur the shutdown time has something to do with the updates, or antiviruse or something. in any case its shutting down properly and i wasnt troubled by it.