Windows 7 (Except for Family pac) can only be installed on ONE computer at a anygiven time. This means that:
1) If computer dies, you can re-install one your NEW computer, This includes the OEM version.
2) If you upgrade your computer, You can re-install UNLESS it is the OEM Version. 3) You can ONLY re-install the OEM version IF the MB dies, You can NOT upgrade the Mother board and re-install.
In the above cases, You may get a message stating somethink to the effect it is not a valid copy, BUT you just need to call Microsoft and explain. They will issue you a New Key code.
If You just need to re-install on the machine that it was already on, ie your HDD died, Then that is NO problem.