Sorry, I'd not trust Kaspers', since It comes from Russia, where the Russian Mafia runs the show.
As for why computers slow down..... it's because they are constantly asked to do more things, handle more files, etc.
Think of it like a brand new pickup truck, that runs real quick when new, but every day you put another cement block in the bed of the truck.
After a while, the truck gets slower and slower and begins to ride like a horse drawn wagon.
The solution to both problems is to get rid of the Overload. It's what I've done for a living for more years than I care to remember.
I not only get rid of the CRAP, but I also tweak and tune Windows for maximum efficiency.
You must go into Startup and the task manager and disable every program that you don't absolutely, positively, 100% have to have running from Boot Up. Then you can go into "Programs and Features" and UN-Install every program you don't need any longer.
I've spent hours doing that, on PC's that had slowed to a crawl. And when I got done, they ran faster than when they were brand new.
Actually a new PC, right out of the box, is kind of a dog, because they are never Optimized for peak performance.
If you were local to me, I'd say bring that dog over here and I'll fix it for you.
Good Luck,
TechnoMage in Florida