Sorry, Cochrane have dealt with vitamin C and echinacea.
Vitamin C is pretty conclusively found to have no benefit for prevention or treatment.
Also no evidence for Echinacea preventing colds, but apparently some evidence for effectiveness as a treatment.
edit: links www.cochrane.org/reviews/en/ab000980.html
www.cochrane.org/reviews/en/ab000530.html
( my other username is a porsche , posting shit pictures so you don't have to, Tue 22 Sep 2009, 8:03, archived)