Is there any connection between:
My computer has slowed down considerably
and
I want to do a reinstall of Windows
?
Do you think that doing a reinstall will make it go faster? Or are you actually proposing to do a reformat?
If you're proposing to do a reinstall, chances are that that won't fix whatever is making it slow. A reformat followed by a reinstall probably will, but that's a seriously drastic step to take - it typically takes me around a month to set up a new system, by the time I've found and installed and configured all the little odds and sods that I only use very occasionally and have forgotten about, and there's no way I'd want to go through all that when I didn't have to!
Now, even after you reformat, and reinstall Windows, and reinstall each of your applications one at a time, and reconfigure everything back to the way you want it, your computer will just slow down again in exactly the same way, unless you identify what you've done to cause it this time and take steps to avoid doing the same thing again.
Why not just take the easier and (vastly) quicker option:
(1) find out why it's got slow
(2) fix it
(3) make sure you don't do it again.
(Yes I know there are people who just throw their PC away and buy a new one when they've installed so many viruses and so much spyware that the thing has become unusable. But that approach isn't good for global warming, now, is it.)