The Japanese word for paper notebook is not ノートブック but ノート (Hence デスノート, by the way). ノートブック was simply too long for Japanese people. Therefore, notebook PCs are naturally called ノートPC, ノートパソコン or simply ノート in Japanese. From my understanding, laptop is no longer common today also in English and it has been almost replaced by notebook.
We almost never use ノートブック to refer to paper notebooks, but when referring to notebook PCs, ノートブック(PC/パソコン) is common as well as ノート(PC/パソコン).
EDIT: Hmm, seeing the comments here, it appears that notebook (PC) is not yet widely used among native English speakers, although manufactures stopped calling their products laptops long ago. Here in Japan, ノートPC has already replaced ラップトップPC almost completely, even among ordinary people. Maybe that's because ノート is far easier for us to understand :)