pc is slow

my pc is slow even after following the instructions defrag, disk clean-up,
start-up, anti-virus, deleting programs, deleting files, history, cookies,
cache...etc.. etc..

this happened after OS was reinstalled. my connection is DSL.

my pc and its peripherals were never cleaned eversince ( 2 yrs) and i was
told we need to clean every 6 months. is it possible this is reason why my pc
is slow?

thanks
.

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