I recommend to use Comodo Antivirus Software .It is really good and safe
Try this link to download http://antivirus.comodo.com/
Prevents all viruses, spywares and
Improves performance and efficiency
Fast cloud-based scanning
Detects suspicious hidden files
Protects all your devices
Yes usually it becomes virus free after installing windows but malicious practices are increasing day by day that’s why for your safety you must have a good antivirus program installed in your PC to get rid of various Trojans, malwares and spywares etc. Some good antivirus and antimalware software are Immunet, AVG, Avast, Malwarebytes, Norton..etc
I would advise you to never use free security from the internet. You are just asking for problems when you install this "free" stuff.
Always use a professional all in one security system such as Norton, Panda, Kaspersky, etc...