Extremely slugish OS

For some time now I’ve noticed my system getting more and more slugish. I’ve performed several suggested maintenances to no avail. At best of times, there would be a momentary improvement, then, the response time would turn to a crawl. Wether in a Application or on the Internet, with all (3) IE, Google Chrome, or FireFox, all the same results.

I have tried to, “de-fragment or clean,” my registry, (at some point learning that that proceedure may cause more harm then, good.

Short of Clean-Re-Install of OS, I am exhausted all my know how.
Could anyone help out?
Thanks.

JB