vitamin c has been shown in trials to be fairly inneffective, particularly if you start while you are sick
the only thing that i would probably suggest to someone as an over the counter preparation is echinacia...though that is also not too effective if you already have whatever sickness youre worried about. as a general immune system booster it is effective enough for me (and the rest of the medical world) to advise transplant patients or others on immunosuppressive drugs not to take it
food, rest, and avoidance of immunosuppressives is more effective overall, by far
cheers