The hard drive may be going bad. Had any shocks or hits to the computer? Once it gets going damage increases exponentially over time and sectors start going bad faster than the system can mark them. It slows down the system because the error recovery on the data takes a lot of processing. Also the computer caches data to the hard drive and when it stores to damaged areas that cache is ruined so it burns CPU cycles rebuilding the lost data that was supposed to let it quickly pick up on a task where it left off.
When my computers mysteriously start slowing and I cannot trace it to a specific program it is the HDD failing.
always a good idea to check the system event log periodically.
windows key + r
type “eventvwr.exe”
select “system”
filter current log, select event level “error”
any version of windows.