We take those packets of Vitamin C called Emergen-C - when we feel a cold coming on.

[Super Orange Emergen-C fizzes in water to furnish 1,000 mg of Vitamin C with 32 different mineral complexes plus B Vitamins. Sweetened with Fructose for Quick Energy with Staying Power.]
many other flavors/combos

http://www.alacer.com/

some interesting immune boosting sites -
http://www.askdrsears.com/html/4/t042500.asp
http://www.shirleys-wellness-cafe.com/immune.htm
http://naturalmedicine.suite101.com/..._immune_health