I have a 13 yr old mac & the files seem to be fine on that. It's never had a re-installation or any kind of change since it was purchased. Still the same HDD.

It is apparently true that all digital storage formats do wear out. At least it's arguably better than non-digital formats however. And they are also susceptible to static, UV, magnetic interference!, etc.

What you do need to consistently do on windows machines is defragment your HDD's. And yes a fragmented HDD will slow your comp down considerably. This was also true for the older macs using older mac os's. But it doesn't seem to be the case for mac osx. There's other issues re: mac osx. Also a windows HDD that's almost full will slow down the system. AFAIK

By the way, there's ways of optimising your system & getting rid of unnecessary files. This may mean start up files (during system startup), registry, etc. Google about this (optimising your system.) I'm no windows expert (or about anything for that matter), so I can only offer simple advice re: your problem.