Windows will realize that hardware has changed and make you re-activate your key which may mean calling the toll free number and telling them your hardware changed. The reason why it is recommended you reformat and start from scratch is because you have a set of drivers installed for your current hardware which wont exist soon and will be replaced by new hardware with new drivers. If you lost the disc it will work but I would suggest you uninstall all the old drivers before making the switch to the new mother board. Also after you make the switch and install the drivers download and run ccleaner to clean up a bit.
It depends on what you get. If you're getting a new motherboard with an identical chipset, you should be fairly good to go. If you get a motherboard that has a radically different chipset (northbridge + southbridge) configuration than your previous motherboard, chances are you'll have to reinstall.
the only drivers you really need to concern yourself with will be chipset and any integrated components like audio or networking or (some) raid controllers. it is not truly necessary to do a complete reinstall. And also all the drivers you will need shuold be included on the disc that came with your new mobo.
I really don't understand how people lose operating system discs. I mean, I can 'maybe' understand losing a game disc or two, though that's never happened to me, but you don't hear of people losing their keyboard, monitor, speakers or other components to a PC. The operating system is about as important as you can get when it comes to components of a PC.
OT, if you have a legit key for a retail copy of Vista, then contact Microsoft and request an replacement disc. It still should only cost you a fraction of what you paid. If your copy was OEM, you'd have to purchase a new copy anyway, so I would suggest a retail copy. That way, you can upgrade hardware til your heart's content, and always be able to reinstall the OS with no issues.
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