I am confident it is only a Windows XP issue. I have had this problem since November and have tried so many different things to try and resolve it. I would say Windows 7 is not affected and it is most likely something to do with the 3D work that was put in to the ESXi hosts which XP does not support. The same environment, same number of VM's and my hosts are not going above 55%. With XP prior to the update that caused the issue my hosts were around 85%, major improvemnt over XP. At this site we use Windows 7 with 3D and no 3D, each behave as I would expect. So on a positive note Windows 7 does not appear to be affected and the resource handling is far superior than on a v5 host running XP. The Apex cards seem to work much better as well on Windows 7, v5.1 than they did with XP, v5 host.
Thanks.