Yes, you need to rebuy. Your agreement with microsoft license term is that the oem copy of window vista is used on one and only on the Compaq you purchase. Unless the Compaq mobo is broken and needs replacement (if Compaq cannot provide you with the original model of mobo, they can give you another one, then you have to contact Microsoft to tell them what happened and MS gives you a new key). So if it was your mobo that die, and you get Compaq to fix it, then youcan reuse. But in your case, no you cannot.