When you put together the available information, preventing colds even in elderly people or people with chronic health problems is improved by taking a few nutritional supplements regularly: In addition to a good multivitamin/multimineral and vitamin D (preferably as vitamin D3 form), adding vitamin E at 200 units/day to the total program seems to help ward off colds.
As with so many tips that we discuss here, it is important to think holistically and understand the need not to use one vitamin or mineral in isolation from others. The human body involves a biochemical network - and no one nutrient or biochemical is more important than another in that they all work together in an intricate dance.
The safest bet is to take reasonable amounts of a multi-nutrient program. For vitamin E, with available evidence, the upper limit seems to be around 200 IU.