Why does the host OS become unresponsive while running installations on win7?

Can someone please explain to me how an application like Parallels can seriously decrease the host system's responsivenes even though the CPU usage is practically at idle?
This happens when I run XP or win7 guest OSes (didn't try anything else) and especially when you install something (updates e.g.)
The recent Apple MacPro firmware upgrade didn't resolve the issue either.

I don't get it, how a rig like this can be forced to its knees even though no real computing power is demanded.