PC does not just refer to a Tower or Desktop unit. It originated with IBMs personal computer and later refers to IBM compatible personal computers. This basically went on to mean any systems with hardware capable of running the windows OS so in short
A laptop running windows is a PC
A netbook running windows is a PC
A Tower/Desktop running windows is a PC
A Tablet "PC" running windows is a PC
and at a stretch seeing as Macs today use the same CPU as windows running PCs you could stretch that most Macs are PCs also
