While in the game hit esc and go to the settings menu and select gameplay. inside that look at sensitivity with a slide bar. Move the marker to the left or right. Test ingame.
OR
Turn vsync off if you have it on. Make sure that IPresentInterval is set to zero in preferences.ini.
From tweakguides.com
bMouseAcceleration=1 - If this variable is set to 0 in SkyrimPrefs.ini, it will disable mouse acceleration. This can provide a consistent and responsive feel through more predictable mouse movements, as the mouse will no longer be faster for larger movements and slower for smaller movements. It can also help reduce the feeling of lag when VSync is enabled by default.
fMouseHeadingSensitivity=0.0440 - This variable in SkyrimPrefs.ini is controlled by the Look Sensitivity in-game setting, and should first be adjusted there. However if you wish to assign a higher value than the maximum allowed in-game, you can do so here.
[Controls]
fMouseHeadingYScale=0.0200
fMouseHeadingXScale=0.0200
The [Controls] section and variables above must to be added to the bottom of the Skyrim.ini file to modify relative sensitivities of the Y Axis (up/down) and X Axis (left/right) of the mouse. Higher values increase the effective sensitivity of the relevant axis.
Howto fix input lag and vertical sensitivity for mouse and 360 controller
http://www.youtube.com/watch?v=g2AJDJ8sOfQ