Dust, young padawan.
Either clean it yourself (if you know what you're doing) or take it to a tech-guy.
Dust blocks the hot air flow inside the computer and the temperatures go up. The fan tries to fix it by speeding up, but if the thing is all clogged with dust that wont do no good.
Clean it frequently (once every 6 months, depending on your local conditions)!
by the way, is it a laptop? Cause 136f on a laptop is not so high as on a desktop.
If cleaning doesn't solve the slowness (the fan problem should be solved) then you also have a software problem. (clean the registry, reinstall windows after formating... etc).