You could already be most of the way through a clean install by now- seriously.
It's the best way to be positive you can trust a system, and takes less than an hour.
Either way you'll have to install your applications and customize it, so might as well reinstall the OS before you invest more time in it if you have any doubts.
Other than that, you can install Malwarebytes, update its definitions, then do a full scan of all internal drives- that's usually enough to find anything malicious. It's got a fantastic detection rate, and is free.