My 12 year old laptop, which I only use for facebrook etc and a few forums, began to slow and all systems were at 100% usage. After a few weeks of it slowing to a stop, I was going to bin it but then I checked the installed apps. Bloody Copilot has crept in and was feasting on the memory, greedy little beast. After I uninstalled it, the laptop returned to its normal speed. I checked my desktop and yes the blighter was hiding in there as well.
I knew there was problem because, at times,the mouse pointer had become uncontrollable. I gave it and the keyboard new batteries but it didn't fix it. After it was uninstalled the mouse was as smooth as a smooth thing. . Quite apart from my normal paranoid pensioner anxiety, in that I know all this stuff is spying on me, harvesting my data and influencing my shopping price options (who knew it could do that?). It makes my stuff run slow so in the end we all have to go and buy new stuff with more and more memory to feed the hungry so and so's.
If your PC starts to misbehave, check it for uninvited guests and evict them.