Saving the state of a VirtualBox virtual machine from command line?
From the VBoxManage manual:
VBoxManage controlvm <vm> savestatewill save the current state of the VM to disk and then stop the VM. (This is equivalent to selecting the "Close" item in the "Machine" menu of the GUI or pressing the window's close button, and then selecting "Save the machine state" in the dialog.)
Then you can start again using VBoxManage startvm "Windows XP" (assuming your VM is named "Windows XP" of course).
You should find the VBoxManage.exe somewhere in your VirtualBox installation folder. Of course you can add it to your PATH to have easier access.