In pre 2.0 VirtualBox seamless mode was only for Windows guests on linux hosts. Now Linux guests on Windows hosts benifit from this invaluable feature. Albeit there are not many instances where a graphical linux program is require without a Windows alternative, but if need be the feature is available.
Just boot up your guest, and hit your hotkey + l and viola, VirtualBox rips the gnome, or kde docks right off the desktop and slaps it into Windows. Frankly it can’t get much better than this, and my main hopes for the future is better linux kernel support further reducing host CPU load when the guests are idle. This will further realize the dream of virtualization, which is the deployment of multiple virtual machines on a single host system. The less host CPU load experienced results in more guest OSs being deployable.
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Seriously I have 4 guest Linux operating system running at the same time, and the host CPU is not nearly as busy as when a single Windows OS is running. Windows literally consumes the same amount of CPU power as maybe 10 Linux operating systems running simultaneously. I use Windows XP in seamless mode to have the benefits of browser compatibility checking, and other Windows only applications; the CPU requirements of Windows is simply tremendous.
Although you can significantly reduce your Windows CPU requirements by turning off the page file system. Frankly on a machine with over 2 Gigabytes of ram, the page file system is simply not required.
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For the longest time I was using Debian for my guest operating system, and was relatively frustrated that the host OS always had a constant CPU load of over 20%. I tried many times to compile a custom kernel to resolve this issue, and in the end I decided to switch to a new OS. At first I though Ubuntu server edition was the way to go, but when I was installing the system there was all these gnome, and other graphical components being installed. What is the purpose of openoffice files in a terminal based OS? Overall Ubuntu JeOS is the way to go! It will only install the base Ubuntu system, comparable to how Debian can be configure during the installation process. The kernel used in JeOS does not cause the host to have a constant CPU load, therefore I am overwhelmingly satisfied with its performance.
If you are experiencing high host CPU load with your guest OS, I recommend experimenting with different kernels, and possible switching to a different OS if you cannot resolve the issue. There is no reason your host CPU load should be high when the guest is not processing any data. JeOS is the way to go 
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