Windows 8.
This.
Windows 8 prevents sales of new computers. The "average Joe" user does not like the presumed learning curve. They do not like the change, and prefer Windows 7 GUI over Windows 8 and its tiles.
How do I know? My second job is on the sales floor of a box retail. In many cases, the only way I can get someone to buy a Win 8 product is advising them how to remove the tablet overlay, and get the start button back. (Some I show the registry edit, but most I point to the utility programs (since when did programs become apps?) out there that do the change for them.)
The Metro interface is a flop.