You should charge him only if you can guarantee that what you do will work 100%. Some people install an OS, and then something breaks, and they don't even acknowledge it's a driver they forgot to install or something like that.
I would charge a flat fee, $40 or so, but I'm a professional 🙂🙂
Don't forget, you're only doing 30 mins or so worth of work - the PC and Windows installer are doing everything else. Do it for $30 or so, but make sure to install all the drivers, etc.