You can clone a hard drive to a new smaller drive if Windows occupies less space on the current drive than your new drive is in size. If you want to copy an installed Windows to a disk for another computer, you will have to get new drivers for disk controllers, chipset and videocard to boot the OS on a new hardware. Note that you will be able to clone an operation system by copying it to a new disk only if you migrate it on the same computer (or two computers with similar hardware).
