Hello Funnhs,
Please read your End User Licensing Agreement (EULA). Technically yes you are required to purchase a new copy/licensing agreement because the operating system is tied to the motherboard. Please reference Activation and Registration of a Microsoft product located here http://support.microsoft.com/kb/326851. There is a phone number located in the Knowledge Base (KB) article. Call and explain what has happened and see if they can re-activate your system withough having to purchase a new operating system. Good luck on rebuilding your system :-).
Stephen Holm, MS