I know EXACTLY what you mean, same thing drove me nuts when i first installed vista.
I mean you keep saying "what in the world could this thing possibly be doing so much?!!"
And i disabled windows defender, prefetch and indexing to get rid of it.
And it did.
On my most recent install, i didnt disable any of them, im not exactly sure how many days it took, but it did eventually stop on its own and my pc doesnt do any of that thrashing out of nowhere or on startup anymore.
Now i do have all my updates including SP1, so it may be one of those that stops the thrashing, all i know is i have everything enabled and my hard drive doesnt thrash at all now.