Many slow PC problems are down to a clogged dusty fan and heatsink, you can normally tell straight away if you find that your fan is running high. Your CPU performance can be running at 100% as your fan is struggling to cool its processor and all vital components are overheating (similar to that of our K-series engine and its 18" fan).
PC's would last longer if the user/owner would just perform a simple and quick cleaning operation, this solves many a slow PC where the user once thought they had some sort of virus.
Your CPU fan may look like this...
