BleepingComputer.com: Options For Avoiding Already Installed Vista

I am looking at a new laptop. Unfortunately, it has Vista installed and I don't want it, but am thinking I might as well hang onto it in case it gets to the stage where you can't do anything unless you upgrade or if I sell it the buyer might want it. I gather from a previous post that to get XP on, I'd have to take Vista off before XP could be put on. Would the reinstallation disk put it back later, if necessary? (ie, take it off, use XP, keep the reinstallation disk) If it was put back on after XP, could it just sit there without causing problems, or would MS' bizarre Gestapo programs kick in?