Do you work for MI6 or CIA? Most of us, who have few secrets, are happy to use a free anti virus, such as Avast, an anti Malware such as Malawarebytes, and occasional use of ADWCleaner. As long as you do regular scans, the chances are small of having problems, and doing regular images and back ups is essential. CCleaner (free) has a very good drive wiper facility and you can choose various options (no of passes, wipe free space, etc)
As for people listening in, why would they want to? To me phones and PC's are tools that take up little time, and I do not worry too much about such things, life is too short, and I have few secrets!