[MidnightCowboy]

I agree and would even go further to include many of the so called "system care" products. As I posted recently elsewhere, from my experiences users often require a "system cure" after using such software. Comodo, Iobit, CCleaner and RegSeeker all fall into this category and I'm sure there are many others.

The biggest causes of system slowdowns are malware infections and general issues involving software installation such as;
too many active security programs
conflicting security software
Badly configured programs
stuff that's not been correctly un-installed
mis-managed system "tweaks"

Insufficient processor capacity or system memory for the required applications to run is also a common problem when users buy a baseline PC and then expect it to run games and similar software.

One of the few "normal" causes would be an un-defragmented hard drive that's almost full.