A brief background, without references:
Many people believe that Vitamin C taken above normal daily requirements can prevent or get rid of colds and other basic maladies. This is substantially incorrect - provided you have whatever your body wants, the rest gets pissed out anyway.
Many people also believe that megadoses of Vitamin C (in the order of several thousand times the recommended daily intake) can cure cancer. This is not only incorrect, but doses that high can lead to kidney stones as the kidneys try to piss out more than they evolved to handle, so should be avoided anyway.
Now this: Vitamin C may blunt cancer drugs, study finds (ABC News Online).
Suck on that, alternative medicine freaks.
The best advice on diet ever remains these seven words:
Eat food. Not too much. Mostly plants.