Laptop gets stuck randomly, especially when HD io happens

Hello,

I have an Asus K53S with a strange issue I can't seem to fix. It works fine about 50% of the time, the rest, it freezes at HD io operations. And by freezing I mean, the process itself can't continue, the laptop is still functional. So, for example, if i download something, the thing remains stuck at 50%, I can use the laptop otherwise, but can't finish my download.

Some other times however, the whole laptop freezes, and that's that.

All this happened after I dropped it several days ago.

What I've already tried:

- a different, working HD: same problem

- different working RAM : same problem

- reinstalling Windows : sometimes it manages to finish the installation but the problem persists, other times it freezes up at 30%- 40%. However, even if it finishes the install, the problem persists on the new operating sys.

Where should I look next?