It's actually pretty simple. Steam only needs Steam.exe to be able to update itself with the rest of the required files. So go into your Steam directory/folder and delete all the files (NOT the directories) EXCEPT Steam.exe. Again, NOT the directories/folders, just the files, and NOT Steam.exe.
You can also use this method to change where Steam is installed by moving all the directories/folders inside the main Steam directory to another location, then moving JUST Steam.exe and running it.
Of course in either case make sure Steam isn't running.
You may want to backup the files you're about to delete in case something goes wrong.