I accidentally changed the esc key to something else, now I can't get into the controls setting to change it back. How do I reset to the default key bindings?
When you re-bound the ESC key Skyrim should have swapped the ESC key with whatever key that action was originally bound to, so you should be able to use whatever key you replaced to access the System Tab.
Alternatively, providing that you haven't re-bound the Journal hotkey, you can press J to open the Journal and then simply click over to the System tab to change/review your keybindings.
C:\Program Files (x86)\Steam\steamapps\common\Skyrim\ControlMap_Custom.txt
C:\Program Files (x86)\Steam\steamapps\common\Skyrim\Data\Interface\Controls\PC\controlmap.txt
To be sure, do not delete this while running Skyrim or Steam. I do not have Skyrim to test it, but what I can deduce from the filenames this will do what you want.