If you have a CoA with a product key, you can download a Win7 .ISO file (of the same version/license) here and use it to burn a installation DVD: http://www.mydigitallife.info/official-windows-7-sp1-iso-from-digital-river/
Then you will have 30 days to activate with the old product key. There's usually a 50-50 chance it will be accepted even though you have a different PC now (motherboard). If not, you will have to call a automated phone system and answer a question or two. If it is accepted, you will get a new license for the old Win7. (Another 50-50 chance) If not, you will be directed to a rep to explain why you want to use an OEM copy on a different system. Usually you can tell them the MB died and they will... look the...