I bought I believe a 64 bit premium system builder OEM for my new PC. My question is if my Mobo stops to work down the road...and I have to get a new one, am I to get a new OS as well?
Simply, yes. OEM installs are tied to the motherboard; if you get a new one, it thinks you've got a whole new computer. You can ring them up to reactivate it though.
