This has been a pernicious problem since Windows 7 came out, but I found a solution that works for me. The problem seemed to be restricted to Microsoft Office files (.doc, .xls, .pps, etc.), but the solution might work for other file types as well. I simply moved the actual document to a different folder than the desktop and placed a shortcut to it on the desktop. Now no matter what I do -- modify the document, reboot to PC, or whatever -- the document stays on the desktop at the position where I want it.