Windows Vista boot time nonstop disk I/O ... goes away after long delay

Question

I have a Dell Laptop running Vista. 4Gb RAM, 500Mb HD, CD player.

On booting, it takes forever (20-30 minutes) to come to a usable state. (This makes it laughably useless as a portable device). The HD activity light seems to be on continuously during this process. The Task manager CPU utilization bar shows about 40% CPU "in the OS" (red) and 30-40% in user processes (green) Digging with the task manager (with "See all processes"), there appears to a process "trusted installer" which is running and chewing up a sort of corresponding amount of CPU.

All that goes away when things finally calm down. At that point, the laptop behaves as one expects. If I reboot, I get to go through the same painful delay again.

I'm suspicious of a previous problem I had. I used to have Visual Studio Express for C++ installed on this laptop (~~ 2 years ago). It used to work fine, including none of the huge boot delay nonsense.

At some point, when a new version of VS came out, I set up to install it. This is a 400MB download. I started having this kind of trouble (huge boot delays; I cannot verify that the above symptoms occurred then). When I went to inspect the Windows Update log, it would tell me that the new install for VS failed. Apparantly what would happen after that is Vista would go and refetch the entire download (dragging the machine down for all the additional network delays), and after refetching the whole thing, Windows Update would schedule a re-install boot. And I'd get same bad boot delay again, sigh.

That continued until about 2 months ago... in desperation, I uninstalled Windows Visual Studio (the old, working one). There is now no evidence that I can see that it should be trying to (re/new/)install Visual Studio; it certainly isn't in the installed programs list. (Yes, I guess I need to go see what Windows Update log says since I haven't looked recently; I'll get back with that data tomorrow when I'm near the laptop again). But since I'm seeing the same delay, I have this feeling that Windows is still trying to install the Visual Studio upgrade.

Any hints on where to look next to diagnose or especially solve this problem would be welcome.

EDIT: Another SU answer pointed to a MS diagnostic xbootmgr for discovering what happens during boot: http://windowsitpro.com/systems-management/diagnose-shutdown-problems-xbootmgr Something to play with when I get near the laptop again.