If I understand your answer, if it is a OEM version, you can't replace the MOB or other components? To make replacments, you have to purchase new software plus the parts??
An OEM license is supposed to be bound to the first PC that it's installed on. The PC is defined by its motherboard. The PC remains the same if every other component is replaced.
However, you're allowed to repair the PC by replacing the motherboard with the exact same model, or one designated by the motherboard maker as a replacement if the original is no longer sold.
If you have to activate the repaired system by phone, it's up to Microsoft whether to give you a new activation code (or not). I haven't tried it, but I have the impression that MS isn't strict about it.
If you're using a System Builder copy of Windows 7 on a system that you built for your personal use, you're violating the license agreement: the system is supposed to be strictly for resale. As far as I know, Microsoft has no enforcement of this. I speculate that most of the System Builder licenses sold by Newegg are used on personal systems.
(The System Builder license for Win8 has changed. For a DIY system, it replaces the Full Retail license, and functions like it. For a system that is sold, it acts like an OEM license. Confusing, maybe.)
