Plz tell best and free antivirus and overall security.?

My Pc is not formated and running without any antivirus from many months.I have various useful and unuseful programmes,documents stored in my laptop means 60% of place is occupied in whole computer and 40% free.I cant store each and everything in pendrive ,i cant buy now ,i have 2 8GBs but not sufficient.Wht to do tell to save my laptop from crashing?