|When you change the motherboard and cpu, the OS will no longer work due to change of hardware.|
At the minumum you will need to boot in with the Vista dvd and run setup choosing the repair option, which may or may not work.
Failing that you will need to reinstall from scratch.
In either case you need to get into bios and set the dvd drive as first boot device.
If your version of Vista is an OEM disk, the license is not transferrable, new m/board and cpu is deemed to be a new system and as such will fail activation.
You can phone Microsoft and explain your problem and they may or may not issue you with a new key.