I have two identical machines (Dell Vostro 200).
Machine A has a dead hard disk, Machine B has a dead motherboard.
Can i swap the hard disk from machine B to A ?
It has Windows Vista Business installed which came with the machine.
Yes, you should be able to do this without a problem, although it may require that you do a repair install. The worst case scenario is that along with the repair install, you'll have to re-activate windows, which is perfectly fine, even with OEM licenses, since you're essentially replacing a faulty component.
In any case, to keep you completely legal, you will want to change the product key to the one that came with the computer that still has the good motherboard. I know that it's in the System control panel in Windows 7, but I don't know if that's the case for Vista.
I'm not exactly sure how the Bill Gates mini-me inside your pc checks for genuity... Even though I've been browsing through their website on activation, but I say put it in and see what happens. If there is a conflict, then contact Microsoft and they are willing to help you though getting the key to work:
This is the above part mentioned.
Hope this helps.