Anecdata is interesting. Here's mine, I've recently had a spate of a few Dell system 3-5 years old getting progressively more sluggish. Quick tests don't turn up issues. When I'm given the time required I use the hard drive manufacturer's testing utility and do thorough testing at which point damaged sectors start to turn up, not too many but enough to cause seek issues or something. I use chkdsk to attempt to recover data and mark the problematic sectors and at least for a while the system are a bit better. Of course, as soon as possible replacements are needed.