I am running Windows via Virtualbox on my Macbook. It works fine. When I switch to fullscreen to screen the screen is actually full, but the windows windows does not spread out to the full extent. How can I fix that?
I would guess that you have not installed the guest additions in your guest system.
Quoting from the manual:
In the "Devices" menu in the virtual machine's menu bar, VirtualBox has a handy menu item named "Install guest additions", which mounts the Guest Additions ISO file inside your virtual machine. A Windows guest should then automatically start the Guest Additions installer, which installs the Guest Additions into your Windows guest. Other guest operating systems (or if automatic start of software on CD is disabled) need manual start of the installer.