As I understand it, MS links your copy of Windows to a particular hardware profile, and this would be especially true for a preinstalled laptop copy which would be OEM windows. If you try to activate that key on your desktop, I'm almost positive it wouldn't activate.
I'm not aware of any great ways to save money on Windows, sorry. Though if you're in college or know someone who is, you might be able to get a student discount (read terms very carefully to understand any limits).