This issue looks like it's been beat to death at this point but let me give you a little info...
1. The Windows license is tied to your MachineID, this ID is generated based on the hardware inside your computer. So when you make any significant changes to your hardware such as changing out your Motherboard the MachineID's no longer match.
2. What you are experiencing is totally normal for a Motherboard upgrade and generally the easiest way to solve the problem is simply by contacting Microsoft, I have not heard of anyone not getting the new key after contacting microsoft (this assumes a legal version of windows obviously).
3. When dealing with prebuilt systems such as Dell, Acer, etc... that have proprietary licensing their license key is generally tied directly to their motherboard, so if you change out the motherboard your installation disk most likely will no longer work.
4. If it comes to the point that you have to buy Windows, I highly suggest you buy Windows 7, and not another copy of Vista. Also as someone mentioned there is a standing student discount for Windows, you can generally have anyone that has a valid Student ID buy a student version of windows at their local university store. ($30 sounds about right for the student price).
5. There is always the Pirate option, but at this point it's generally not worth the hassle and the likelyhood that you will get a virus while using one. More than likely if you contact microsoft they will help you right over the phone without having to purchase anything or risking your computer to a virus.