http://www.techradar.com/news/the-best-free-pc-optimization-software

Is your Windows PC running a little slowly? It's not your imagination: over time hard drives accumulate all kinds of things that slow things down. PC optimization software can't solve obsolescence – if your computer is struggling to meet the system requirements for your OS or apps that's a job for more RAM or the addition of an SSD – but if the slowdown is software-related, it can really help.

It's worth noting that you can do much of what these PC optimization apps do yourself using Windows' built-in system maintenance tools, but that's time consuming; what's really being sold here is convenience.