Bad idea
If it's been years since the PC was bought, the material used to "just" re-install will created a dated and exploitable code base that will be tricky and tedious to safely update and secure.
It may not even be safe to install (e.g. if OS installation material pre-dates 137G safety on a PC that now has a larger hard drive) and because you've done nothing to exclude bad hardware, the result may be an unbootable mess.
For examples, consider a failing hard drive, or bad RAM. A PC that has had bad RAM for a few months has a 95%-OK code base before you "just" re-install Windows; now it has 100% of that code base written through the lens of bad RAM.
So even before you consider the adverse impact on the user (which you may consider punitively acceptable, if you've become sufficiently burned-out and cynical), this is a bad idea. Certainly not a service I'd pay for.
