|I agree with jam.|
To answer your question though, you wouldn't be able to recycle the operating system on a new build. The OS is keyed to the motherboard. Sometimes very similar system from the same maker will skate by but you are talking about a totally different animal.
If you don't want to pay a ton for your OS then you will need to buy an OEM computer like you currently have. HP, Dell, Gateway, Acer, etc.
Look at the link below paying special attention to the New OEM restrictions.