Is it helpful to take vitamin c before the winter to prevent from colds ... ?

No: Any benefit from vitamin c comes at high doses. Imo, it is not worth the trouble.

In some situations: Research shows in most situations, taking vitamin c does not prevent colds, but in those who do extreme physical exertion or are exposed to cold temperatures it's a useful preventive- see http://www.Ncbi.Nlm.Nih.Gov/pubmed/15495002?Dopt=abstrct . Studies show vit c reduces duration of cold symptoms. High doses at onset may be especially helpful- see http://www.Vitamincfoundation.Org/surefire.Shtml.