The symptoms of an incorrectly set themes folder is that the EE Control Panel loses all its styling - the menu becomes an unordered list of blue underlined links etc.
Did you change the name of your themes folder? if not then this fault will almost definately be caused by an incorrectly set path to your themes.
Navigate to Admin -> General Configuration and check that both the "URL to your themes folder" and "Themes Folder Path" are correct.
As with the templates folder we like to use relative paths for the server path if possible but if you're struggling then make sure the full server path is entered in here. For Windows installations use forward, rather than back, slashes for the directory separators in the path (Windows doesn't mind which way around they are).
We've also noticed that it is important to incude the trailing forward slash on the "URL to your themes folder".