Here at one time MS was selling the OEM version of XP if you were doing a hardware upgrade, so as to reduce the amount of pirate copies of XP ( 1000 computers, one copy of XP on them all with the same key). Discounted, so you would go buy a MS Intellimouse, and get a Cd and COA for a very low price, provided it was for a PC you were not using yourself. So take a friend to pay with your money..........
Did toss away some Win98SE packs away recently, never used and never opened, but bought just in case we needed extra PC's and wanted a key for them. Just bought enough copies so we dropped to the bulk price, was around 25 copies IIRC. Of course doing the assembly and install it was a lot faster to Ghost a single drive rather than run the CD, so was busy for a few minutes testing each assembled unit doing some quick regedit work to put in the key as a initial Qc check.