3 or 4 Years
If you average the FOUR years of patent licenses/litigation costs over the THREE years of installs, then yes, its about $21.50. If you assume windows sales were constant over a FOUR year period (at the same level as before), then the cost per user drops to $16.10.
Does the author explain the mismatch in time periods? If not then this is a pretty poor piece of research. I could equally claim that MS is amortising $4.3b over all the copies of Windows Vista sold today (or over all the Windows Vista sales in China... http://www.newlaunches.com/archives/only_244_copies_of_genuine_windows_vista_sold_in_china.php).