with oem its 1 motherboard for the life of the licence, if you change boards then you need to go out and buy another oem copy, yes you can phone up but that is breaking the EULA and will void all support from ms
here is some clarifaction from BFRD link
I am tired of reading so many posts suggesting that you may install an OEM copy of a Windows OS on multiple computers and still retain a proper license. Just because you call up and ask for a license key and you are given one does NOT make it OK. Not getting caught is not the same thing as being allowed. The operators that are manning those phone lines do not have the training (or inclination) to determine if you are installing on a new machine or not. The license is clear; OEM = one machine only.
OEM licenses are cheaper for a reason, and it has very little to do with not having a box. It is cheaper because eventually you will need a new machine and will then have to buy another copy of the OS. So next time someone suggests that installing an OEM OS on a new (read: not the original) computer is OK; will get a Piracy Infraction.
Minor hardware changes are permitted
A motherboard change is never minor
Hardware failure may not allow you to re-activate a license
This thread is about licensing not morality
MS will often allow you to re-activate even though they shouldn't, they have stated this will not protect you from prosecution
If you are upset about the licensing issues of OEM software complain to your system builder