CPU usage shoots to ~50% and stays there until suspended

Solution 1:

Take a look at the Windows Performance Toolkit: http://blogs.msdn.com/b/pigscanfly/archive/2009/08/06/stack-walking-in-xperf.aspx

My money's on crappy drivers.

I had this happen with crappy Broadcom (that's redundant) network drivers.

Solution 2:

I would suspect incorrect/bad drivers, flawed BIOS, or outright failing hardware in that order. It's very, very unlikely that this is a problem with Vista itself. Interrupts of the sort showing up in Process Explorer are the result of programs having non-fatal errors or deadlocked conditions while running in real mode. At the top of my list would be Sony's drivers for the swichplate buttons (those above the keyboard) and the keyboard special functions. Having worked with Sony's software before, I can honestly say that it's total crap.

Solution 3:

I would try to turn off unwanted Windows services, and most notably Windows Search.

The most authoritative list of services that can be tweaked is on Black Viper's Website.

You could also have a look at TweakHound's Vista Services Recommendations.

Create a system restore point before starting. You could also use Autoruns as your tool, since it can save the current configuration on a text file and restore it later.