Many many places provide guides on how to clean up computers and to make them faster. This is a method that I personally use and it's used by technicians on the field to get clients' computers up and running in as little time as possible. The items are shown in order so just download each one, install it and run the applications (note that all these programs are free). I hope that this will provide much help and if you have any questions feel free to comment.
1) CCleaner - A system optimization, privacy and cleaning tool. It removes unused files from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. Additionally it contains a fully featured registry cleaner.
Site: CCleaner.com
Download Link:
CCleaner Download
2) Malwarebytes - Malwarebytes' Anti-Malware can detect and remove malware that even the most well known anti-virus and anti-malware applications fail to detect, just make sure it's up to date. Do a Full system scan with this program. The quick scan option works well but once in a while it will fail to find some things that the full scan will.
Site: Malwarebytes.org
Download Link:
Malwarebytes Download
3) SuperAntiSpyware - A very good program that removes spyware, trojans, worms...etc. It does similar work as Malwarebytes but here and there it picks up a few things that Malwarebytes left behind so I recommend it being used after using Malwarebytes. Just make sure it's up to date and that a full system scan is done as well.
Site: SuperAntispyware.com
Download Link:
SuperAntispyware Download
4) Defraggler - A Disk Defragmentation tool that does a great job. Just run this last and then restart your system and your PC should be in good shape.
Site To Download: Defraggler.com
Download Link:
Defraggler Download