There are several possibilities:
- Buggy drivers
- Buggy windows
- Buggy hardware
- Buggy end-user software

I answered that:
- Disk fragmentation
- Bloated registry; this doesn't mean it's corrupt, just that it contains a vast amount of (very often unused) entries.
- Unnecessary background programs

You can only show that there's a virus, there's no tool to show that there is NO virus. Well, there's a way but it is hardly practicable.

10 years sounds reasonable for a harddisk lifetime. 20 years sounds risky. The problem is that old hardware from the 80s or even older often works fine. Most electronic problems are caused by bad capacitors and sometimes fine rifts in electronic boards. Old disks and tapes stored data less dense and the used materials were better known and understand than some of the modern materials used. Usually, old hardware and media are much more robust whereas high-end stuff is comparatively sensitive.

However, especially harddisks are highly "intelligent". So you can monitor them and most of the time replace them before they fail. This feature is called "SMART" and there are certainly tools for Windos which can use this to diagnose problems and tell you the current status of your disk. Of course, that helps with disks in-use only.

If your drive supports power management to turn the drive off when not in use, you should disable it by all means. The most critical event for a harddisk is spin-up/spin-down and stresses it the most. There's a very simple rule that applies to almost all materials: Frequent drastic temperature changes
make things brittle and thus destroy it in the end. A drive that runs 24/7 is much less stressed than one in a computer which is switched on/off several times a day.

Every write-access can (in theory) cause damage to the files due to the reasons given above and explained by others. You can detect this only by keeping checksums for every file around. For example, I always take the time to compare a burned CD with the original data on disk after I once burned some CD-RWs with a defect surface without getting any error from the burner.
I don't see any evidence that mere read-access could cause anything like that.

If you really move files from one folder to another in contrast to moving them to a different partition, coruption should be fairly unlikely as this causes only some changes in the directory entries. That's why this operation works in an instant.

In any case, it's by far more likely that you use data by viruses or accidential deletion than by "bit rot".