Second off. I'm not sure if this is the right forum section to post this in, so feel free to move it.
Anyways. I've had a Compaq computer (ew) for a few years now, but that recently died. (PSU problem, but was bad overall anyways) I built myself a nice new machine a few months ago, but I kept the old compaq Harddrive to avoid redownloading/installing everything.
Anyways I've decided that any remnants of the old computer needs to go, and have decided to get a new harddrive.
My question right now is, do I need to rebuy windows vista? or can I leech the code off the old computer and continue to use it.
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.