Something that im often asked by friends etc is if they are allowed to replace components of their laptop under warranty without invalidating it.
I'm pretty certain that there are laws that states that modifying one component, may only invalidate the warranty on that component, not the product as a whole. Eg if you changed the RAM, then the warranty would not cover the RAM, but it would still by law have to cover everything else.
Is my interpretation correct for the UK, and also for other countries, and that it is therefore safe for people to replace hard drives, RAM, etc themselves, and that doing so will not invalidate there warranty.