Everyone knows that Windows and most programs create temporary files for your use. However, not all programs erases the drafts created, leaving the temp files in user's system.
Temporary files take up space in HD, like any other file. By themselves, they do not reduce performance. But gradually, as more and more useless files are getting stored, the computer begins to slow down because of the fragmentation of data in HD. Besides the wasted space that can reach several gigabytes of hard drives of those who never cleans temp files.
You might think, "with huge hard drives, no need to be cleaning up temp files." Not exactly ... The more space and more time to use your HD, the more it is getting temp files.
Moreover, many programs saves their registers and don't clean it. The registry is a database on Windows, whose purpose is to store, in an organized way system settings, from Windows itself, and programs that want to use it.
There are different sections of registry for each program, but many do not clean their settings and data when they are uninstalled. After some time, this will leave the registry full of settings of old programs you no longer use. For this reason it is good to clean your HD from old registry, but this is practically impossible to do manually.
To start, the registry is not an text file is an binary file that follows an specific format. Windows has an editor, well known: the "regedit". But there are so many places that can store old or unused data, the manual search for them is not a good idea. For this we need to use registry cleaners applications as well.
Recommended applications: CCleaner and RegClener
I based my answer at this site (sorry it is in Portuguese): http://www.guiadohardware.net/artigos/limpando-temporarios-windows/