Windows is a sloppy program. It leaves debris all over the place. Some of those files are related to program activity and other files are left-overs from programs you no longer use. The result is that many users are forced to reinstall their computers every so often. The Windows operating system has something of a half-life. Every six months or so, the system ends up being about half as fast and reliable as it was at the previous six month mark. After a few years, the system becomes unusable. Many people go out to buy a new computer with the assumption that their old hardware no longer works. Most often, the problem is all the garbage Windows makes. CCleaner is a free program that can lengthen the amount of time it takes Windows to reach its half-life. While I have not used CCleaner, I have used programs such as Clean Sweep. Clean Sweep use to be an independant program but it now appears to be given away by Microsoft. Here is Microsoft's page about Clean Sweep.