CPU can't be upgraded, or would be very unlikely. If possible, you'd be limited to PIII's or Celerons.
HD is pretty much unlimited as long as it's IDE and not SATA
yeah little to no changes can be made. what about a tablet pc, that would be really cool if 1 had the capabilitys(excuse my horrid spelling) to run something like WOW or any major game for that matter