VirtualBox

Virtualbox is software that allows the user to run a Guest Operating System inside of the Host Operating System, without having to reboot.

Installation
The useflag 'qt4' is for graphical interface.

Running
Users will not be able to run and use Virtualbox if they are not a member of the "vboxusers" group.

Changes will not take effect until the user re-logins.

Microsoft Windows Guest

 * Do not enable APIC for Microsoft Windows guests! (VirtualBox also has a bad habit of enabling this option at it's own will.)
 * If you elect to use SATA controller, it is recommended to use Intel's SATA 7.8 driver version (ie. iata78_enu.exe). Using other versions will cause performance problems along with blue screen errors!
 * Slow performance using SATA driver? Only use the SATA controller interface for the hard disk.  Remove any CDROMs from the SATA controller and place them onto a IDE Controller.
 * When installing the VirtualBox Guest Additions into your Windows operating system, do not select to enable Direct3D (experimental) option as this will cause resizing problems and other anomalies. Also, you need to install the Guest Additions from Safe Mode.

After performing the above, you should see blazingly fast performance of the Windows operating system guest within VirtualBox on Linux.

Linux Guest
None!

Links
Virtualbox Homepage