Skyrim has an Almost-Auto-Aim-Assist feature. For archers, you can tweak this (no idea if it actually disables it) via the ini.
C:\Users\xxxxx\Documents\My Games\Skyrim\SkyrimPrefs.ini
Find the Combat header. At the bottom of that, paste this:
f1PArrowTiltUpAngle=0.0
f3PArrowTiltUpAngle=0.0
This will, apparently, make aiming manual.
http://www.gamesas.com/index.php?/topic/1269030-tweaking-arrows-to-behave-more-like-oblivion/