People tend to go overboard with the POWER of vitamins. You're likely getting what you need from food already, but taking a multi-vitamin won't hurt.
Vitamins A, D, E, K are stored in the liver and the only vitamins you can "overdose" in a healthy adult this is faily rare.
They can build up in your system as they don't dissolve in water. Again if you're healthy and aren't taking 1/2 a bottle of multi-vitamins daily this is something you really don't need to think too much about.
Vitamins B and C are filtered by the renal system (your kidneys).
Mega Does of Vitamin C (or any other water soluble) will cure your cold. The majority of research shows it does nothing. The placebo effect is a powerful thing.
One big exception is B-12 for vegan or vegetarians patients. Found in meat, dairy and soy it's recommended to take a B-12 supplement.
Your stomach produces a protein called gastric intrinsic factor (GIF). GIF is required by the small intestine to absorb B12. Without this your body cannot create red blood cells.