As you have just discovered, it is quite common for laptop screens to have a dropped pixel or two. It is not really PC World's fault (I can't believe that I am defending them). Tapping them can sometimes free them if they are just stuck rather than having a dead transistor.
Some TFT manufacturers do operate a zero tolerance on dropped pixels now, but many do not.
It is unfortunate that you got a duff power supply 1st time, but I think they have been pretty reasonable with you.