This is dependent upon which version you bought. Factory OEM license cannot survive a mobo change except same model, unless Tech Support helps you do this.
All Windows 7 retail versions except OEM can migrate to whatever hardware you want, as long as Upgrade version has a qualifying XP/Vista for that machine or is retail and not being used.
However retail OEM is locked to the first mobo installed upon and cannot migrate further than that mobo. We get some reports that MS phone activation will make exceptions for good cause (mistaken install, mobo dies soon after install, etc) but you shouldn't depend upon this.
If you need it because failed activation does not refer you directly: How To Activate Windows 7 by Phone Step-by-Step Guide