How do you know if somebody is hacking into your computer?

.I restarted my computer because it was working kinda slow than when my desktop finally loaded everything was gone all of my pictures were in the trash my desktop changed and all my notes from school were deleted along with microsoft word. all my pics from picasa was gone but picasa was still there and aim was still on my desktop but my saved sn wasn't. It was like i never touched my computer.. when i opened my trash i restored my pics.. but everytime i reset my comp they are back in the trash and i cant restore my notes because theres no microsoft word anymore and all my notes from one note are permantly gone-- is there someone controling my computer?