This is a weird one I haven't seen on the forums. After shutting down Vista and quitting PD4, prl_vm_app stays in memory, using 60+% CPU. Got to manually quit the process. Vista is a boot camp partition accessed via Parallels. Also running XP in a VM and it does NOT exhibit this behavior.
Dear pegasus416, please, remove Parallels Tools from the virtual machine, shut it down and delete. After that create new machine based on this Boot Camp partition, install Parallels Tools anew and see if the issue reappears.