It doesn't necessarily mean anything is wrong with it from a hardware perspective.
With Windows, every time you add or remove programs they make changes to the system's registry. Over time, it can be changed in ways that confuse it, slowing the computer down. Thats normally what is going on when someone says their "old" computer is too slow now.
Doing a system restore doesn't really fix this - it just removes the last few changes up until the restore point was created. If you really want your PC to run like a brand new machine you need to completely reformat your hard drive and reinstall Windows.
My best practice for this is to have one hard drive for Windows, and other drives for files I don't want to lose. I reinstall Windows every 18 months or so and its like I bought a new PC every time.
A while back wwhen I reinstalled I got everything set up just the way I like - a/v codecs, software, drivers, colorspace, etc, and make an .iso of the entire drive. Now when I run into problems I reformat the hard drive and load that .iso file onto it - a fully configured pc without needing to reinstall anything.