First, check the RAM on the suspect computer. It is the least likely cause, BUT the most dangerous because bad RAM can permanently damage disk data... all disk data flows through RAM in both directions.
Then, check the hard drive itself.
Depending on the hardware, it can be cheaper to just replace the RAM and replace the hard drive than spend hours agonizing over suspected problems. For example, a high quality 1TB SATA drive costs about eighty bucks.
It goes without saying that a regular machine backup is being taken, with verify. I am becoming a fan of Acronis True Image 2016... I think when they get the recovery kinks out it will be a best-of-breed utility.