Simple, pull out the HDD (harddisk/harddrive) and keep it for data backup. It is up to you if you want to be nice or not and supply a used/refurb HDD you can buy cheap like here for $10:
http://www.microcenter.com/product/485410/Assorted_250GB_7,200_RPM_35_Desktop_Hard_Drives_(Refurbished)
But it is after beggars can't be choosers right? So they can just get the machine and deal with the rest on their own. They can do the work of windows instatallation and all. After all the windows 7 license sticker is on the case right? Let them spend time and effort to get things the way they want it and not waste your own.