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#21
In short, there are 2 different reasons for a system slowing down:
- hardware
- software
So, start with the hardware stuff first (IMO) in order to rule it out. I'd start with these free diagnostics: Hardware Diagnostics
CHKDDK checks the file system on the hard drive - so it's possible to pass CHKDSK even with a problem hard drive (not often, but it's still possible). Running the Long/Extended hard drive diagnostics will help to sort this out.
As for the software problems, try looking at Resource Monitor and Task Manager (along with Event Viewer and MSINFO32) in order to see if there is a particular program/set of programs messing things up.
Beyond that are the different Windows tools for fixing Windows problems (SFC.EXE, DISM, System Restore, Refresh, Reset, or just doing a complete wipe of the hard drive before installing Windows).