When the virtual machine is not running execute this command
VBoxManage modifyvm XP1 --memory 1024XP1 is the name of my guest machine, so replace with yours. The memory parameter takes the amount in MB, in this case 1GB
The first time I ran it I received the following error message
ERROR: The machine is not mutable (state is Saved)
Last time I turned off the guest machine I had selected the save state option. You need to Power the virtual machine down before you can run this command.