measure performance in OS in  VMWARE ESX

I have virtual center 2 and esx 2.5 in my environment
When I look at the performance of a virtual machine in the virtual center I see that the machine takes 4 ghz CPU but when I login to the operating system I can't realy sea that the CPU is working hard and I cant tell whay do I see different thing.
And more of that the operating system has at avarade 8 percent but in the virtual center it takes 4 GHZ CPU witche is 50 percent of my esx server

Does any one know how I can measure the real performance of my OS and my processes inside the virtual operating system?