I've been taking Boots delayed release Vitamin-C & Zinc tablets, as mentioned above a good diet should provide all of that, but often in UK we can be slightly deprived of some vitamins/minerals. I don't take them everyday as Zinc can have other effects, but some science has shown that C+Zn can reduce the duration of a cold by a couple of days.
The 'n' days resting are recommended as a heavy cold is demanding on the body, sometimes compared to strenuous gym work, hence rest should be respected and not do 'double' effort.