I would build people systems for $100 flat labor plus the cost of the parts, and I did probably a few each month on the side at one point in time. I would go to their house, show them how to mount the mobo, install the processor, etc. Then load the OS, total time about 3 hours of labor which is like $33 per an hour at my $100 flat rate.
Also, there is no way you are getting a legit copy of Windows for that cheap. Even if it is from MS and not pirated I am sure you are breaking several OEM and System's builders EULAs and contracts.
People just don't get it. There is no money in building a computer, all the money comes from support. Any idiot can assemble parts in a box. Plus, people look at specs and they will see your specs and compare them to a computer in the paper at best buy or microcenter and see the same thing for half price.