Given Qowys and justinnds posts above, the problem could be one of the mods you have installed. If you have nice install packages, you could try to uninstall them, get Steam to repair your installation and then reinstall the mods one by one to see which mod gives the problem.

The repair option can be found by opening your steam library, right click on Skyrim and choose Properties. This opens a window. Go to the Local Files tab and press Verify integrity of game cache ...