Regime (noun): A form of government: a fascist regime. A government in power; administration: suffered under the new regime. A prevailing social system or pattern. The period during which a particular administration or system prevails. A regulated system, as of diet and exercise; a regimen.
Regimen (noun): Governmental rule or control. The systematic procedure of a natural phenomenon or process. A regulated system, as of diet, therapy, or exercise, intended to promote health or achieve another beneficial effect. A course of intense physical training.
Some people insist that “regime” should be used only in reference to governments and that people who say they are following a dietary regime should instead use “regimen”; but “regime” has been a synonym of “regimen” for over a century and is widely accepted in that sense.
Source: Answers.com / Common Errors in English