Resize with VirtualBox, the manual
http://www.virtualbox.org/manual/ch08.html#vboxmanage-modifyvdi
the command
VBoxManage modifyhd .VirtualBox/VDI/Windows_Vista.vdi --resize 25600
25600MB = 25GB and then resize with Windows 7 oder Windows Vista
http://www.linuxforen.de/forums/showthread.php?p=1534893