Doing a fresh install won't necessarily remove any mods (neither will deselecting use mods, if people have overwritten core files or graphics).
First, go to your install dir and delete the /Mods/ folder.
Then go to c:\users\your_username\my documents\my games\elemental\ and open each of the folders and delete everything inside
Some people may have put assets in the /gfx/ folder in your install dir as well, so while you're at it, delete that folder too
Then open impulse and shift-rightclick on elemental and select "verify installation". when it says there's an update, download the update.
edit: if you're just looking to disable all the mods and play vanilla, the simplest way would be to de-select "use mods", then verify the integrity of the installation. There might be mod files left in your install dir, but they won't affect anything.
