I see this all the time. Blow the dust out of it. That's a big reason why you hear the fan running more now. Dust clogs the fins on the heat sink for the processor which cause it to be less efficient at cooling the processor. In turn the fan has to work harder to cool the processor. What happens is the processor will run hotter than normal and it will throttle itself to help prevent overheating.
Take the side panel off and use a can of compressed air to blow the dust out of the computer. Be careful and keep the can upright as you don't want to shoot any liquid out of the can. If you're blowing into a fan, make sure that the fan doesn't spin too fast as it can damage the bearings. Make sure you get into every little nook and cranny in there and blow the dust out of the power supply as well. It's best to do this in a well ventilated area.
+1 on the Malwarebytes and CCleaner
Also download and install MyDefrag
. This utility will automatically defragment your hard drive on a regular basis, helping maintain performance.