ok, imagine while booting up an operating system, starting a graphics engine as complex as explorer, and copying most if not all of your operating system into ram? or virtual ram?
all at the same time.
sounds painfull. thats why you have a 200mb cache from your virtual memory. one word. fonts. if you have a stack load of fonts installed, its going to slow your boot time considerably, as its loading all these things into ram, and paging into virtual memory.
have a look on
www.tweekxp.com for ideas on how to optimize your system services etc.
as for the network thing, the network services tcp stacks (browse master, etc) stuff only takes about a few meg of ram
and the USB stack can fit in a meg or so too. so nothing really to worry about....