recently I have installed new Xubuntu on one of my physical servers. After that I have installed VirtualBox and Windows Server 2003 as a guest operating system. I have added 4 cores and 2 GB RAM to the guest machine. Now there is a problem that CPU usage on the guest is hardly always 100% and CPU usage for the process of my virtual machine on Xubuntu host never exceeds 15-16%. Is there any cpu usage limit for individual process? Or what could be any other reason?

Thanks in advance for help :-/