The best free software to clean computer is 'Ccleaner'. There are option to clean temporary files as well as fix obsolete registries. But even if you delete temporary files etc, some times computers may not get the original speed. Ccleaner has an added advantage of wiping free space. Even if files are deleted, it will remain in the computer and the only way is to wipe it. Ccleaner will wipe these files with random datas and by this computer will become fast. But please note that depending on the temporary files in the computer, wiping will take time around half an hour or a little more. Wiping option will be in 'tools'.
Why people insist to say these quasi-cliche phrases as "Google is your best friend". Lots of people recognize the power of Google, but they come over here to find quick, precise answers, more or less, from the MDL experts and users. Please stop spitting those sentences so annoying...
Defrag, use autoruns to stop running programs in background unnecessarily , maybe turn off disk indexing, make certain you have adequate free space on your system drive .... Wiping free space is unlikely to speed things up. Your goal is to lower your computer's overhead burden. When too much is running in background, that eats up system resources. Additional RAM might help, but one of the worst offenders is a boatload of programs that start up in background that are killing system performance. You can also try stopping unnecessary services. But be careful here, Some are best just left alone.