A good start in cleaning your computer of nuisance or malicious software is to first remove suspicious programs.
You can also use PCDecrapifier or similar programs to try and help you automate the removal, but in practice doing it manually is the best way.
Malicious programs often hide out in user and temporary directories to evade detection or removal. You should use a program such as CCleaner or Bleachbit.
Malware often redirects your default search engine or installs various tracking plugins to your browser, to get rid of these try resetting your browser.