I assume your building them to order so,
Discuss with your client beforehand what they want, they may already have a copy they want to re-use to save the expense of a new O/s,
Install the preferred windows system using back up/recovery disks,
inform the client their choice of windows O/s is installed but will require activation, then its up to them whether they hook a copy or buy a legal license/re-use their own key.
I give the option of an O/s in the quote so if they want legal win7 64bit, I buy and install that and pass the cost on, but they know about it beforehand and its their choice.
there are ofc other ways to go about it, but I find an upfront, honest approach works best for me