depends what is the problem with your pc. If you suspect it has spyware then install Malware bytes (free of charge). It is one of the best in the world. Secondly if you are looking for installing Antivirus then go for AVG 8.0 (again free of charge).
Hope this helps
I think I have some kind of spyware because I searched for knowledge about spywares for my schooling and noticed that my computer have a lot of described symptoms (slowness, my browser disconnects often and when turning off I've sometimes noticed that the computer shut down a some kind of strange software which I have never opened, called □□□□□□w* or something).
Does AVG or Malware bytes work with F-Secure? Because I don't want to uninstall it.