Hard drives are pretty easy if it's just a 2.5inch IDE jobby, they're usually mounted in their own, for want of a better word 'enclosure' which can be removed with minimal intrusion to the machine and then the drive replaced. With 2.5inch S-ATA drives as long as you can physically access the drive it should be ok, although the only machine I've seen one of those in was a MacBook Pro so I dunno how much they're used in windows laptops (Or how easy they are to get hold of). CPU & graphics are usually a no no, even if you could upgrade them there's the problem of extra heat for the laptop's cooling system to handle.
