Provided that version of windows is not an OEM copy (the type that came with a new computer) then you can.
If you have not activated the original copy for quite some time, it will take the activation on your mac without any complaints, but if you had, then you'll need to call M$. When an activation fails, there's a dialog box that pops up with a phone number, use that to explain to them what your trying to do.
Of course you know that you cannot run two copies of windows (same license) at the same time and if you choose to install windows on your mac you'll need to uninstall it on your original computer.
Check out newegg.com if you need to buy a additional copy of windows, they're generally cheaper then most places.