How do you know, exactly ? Unless you are running a weird operating system where most malware cannot work (say, a customized version of QNX), possibly on customized hardware as well, or unless you never put any media on your machine and never, ever connected it to the Internet, there's no way on Earth you can know your machine is NOT infected.

You can know your machine is not BLATANTLY infected by COMMON malware. Which is a completely different thing than being sure you are not infected at all

You would be surprised at how some things out there are nasty, stealthy and quite good at tricking people into infecting themselve.