When you first run Windows in VirtualBox, some features won't work correctly. The keyboard and mouse must be "captured" to work in the virtual machine and must be "released" (by clicking the left-hand Command key on your keyboard) to function in Mac OS X. Screen resizing, network connections, file sharing, and other features won't work either. To remedy these problems, install the special VirtualBox drivers for Windows, known as the Guest Additions:
Choose Devices > CD/DVD Devices > Unmount CD/DVD Device.
Choose Devices > Install Guest Additions.
The Guest Additions disk image mounts. The installer may run on its own; if not, choose Start > (My) Computer, right-click VirtualBox Guest Additions (D:), and choose AutoPlay from ...