I weren't sure what you were getting at, but yeah, I know that much. However, that's certainly not the only thing that can cause slowdown on a Windows computer. Windows tends to become slow over time, for many potential reasons. It may be a buildup of background processes; this would especially affect older computers, or it could indicate a severely fragmented pagefile, or about a thousand other things that Windows tends to slowly screw up over the years. If you want to test to see if it might be the hard drive, I'd recommend using the Hitachi Drive Fitness test (scroll down; it's freeware, here's a link to a bootable iso); it's usually pretty good at checking whether a hard drive's about to fail. If the computer supports booting from USB, you can just stick it on a flash drive, but if it's older (which, from the description, I suspect it is), you might need to burn it to CD. Don't try to fix the drive with the tool unless you've already backed up (really, it's better just for diagnostics anyway).
On the other hand, if you know the hard drive manufacturer, you might want to try their specific diagnostic tool; it might even end up fixing any problems.