I find it very rare these days the need to reinstall windows. Run malwarebytes in safe mode, maybe use eset online scanner if its a serious infection, run hijackthis and remove any extra cruft and done. Seriously, virus and malware scanners are pretty effective if you use the right ones. I hardly have to google infections anymore for instructions on how to manually remove them.
Or I could spend 2-4 hours reinstalling windows, office, drivers, trying to get various program settings the way they were before, and inevitably miss something and get a call later about how such and such is in a different spot/isn't the same as it was before, and a not so happy customer.
The only times I have actually needed to reinstall windows for a customer is if a bunch of system files are damaged and the thing won't boot. And the thing about windows getting slower over the years is hardly noticeable in my opinion. Most computers just need the shit cleaned out of their startup entries/more ram.