More About That Annoying Bug
I mentioned a few days ago the problem with my wife's PC. Using Process Explorer, I identified that the svchost.exe that was gobbling all the resources was launched by a device driver for an HP digital camera--one that we never connect to my wife's computer. I then removed all HP device drivers from her computer except the one for the HP Photosmart C6280 printer. And guess what? Now it works perfectly!
I don't know if the problem was that the device driver as delivered by HP had something wrong with it, or if a virus had somehow attached itself to it. But Process Explorer is a very useful tool!