Windows Defrag is a rebranded version of Norton Defrag. Windows Scandisk is also an older Norton version of the same product. Hell, most Windows system tools started out as Norton tools. Anyway, the point is that Norton has good system tools -- if you don't have to deal with the bloatware they include with 'em, that is -- so the defragger should at least be decent.
My guess is that you're connected to some kind of LAN with shared devices, or which had shared devices, and Windows is attempting to reconnect to each and every one of these at startup. Same thing happened to me at work, and due to the network settings it would take forever for my computer to boot until I pestered someone from IS to go into the admin window and change the settings.
It also could be some of the background processes XP likes to run. I can't remember where exactly -- possibly Annoyances.org -- but there's a good list of services you can disable in XP to make it run a lot smoother.