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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.