Windows 7 Pro, OEM, $63.00
Microsoft Windows 7 Professional SP1- 1 PC - License and media - 64-bit
I'd just get the OEM, its the same software as the retail, without the fancy box and reduced tech support. If you feel guilty about not being a "builder", then write a bill of sale for the PC to your wife/kid/friend/whoever for $1.00, with a 100 year lease back to yourself for $1.00, then if the MS Gestapo knocks on your door demanding papers plz, you have that to show them.