What I have found works well for me in combating the common cold is that as soon as I start feeling symptoms, I take every 3-4 hours (and normally not more than 3-4 times per day) the following :
Vitamin C : 1g-3g
Echinacea 500mg (one or two capsules)
Zinc 20mg (normally zinc picolinate)
Note : I usually take Ester C, so the amount of Ester C I take is around half that amount, though the equivalent Vit C, I believe is the same.
Normally with the above regime I get rid of the cold within 12-24 hours.
There have been times when it has lasted longer, though this seems to be when my body is in need of a good detox.
Also believe 1g of Vitamin C per day works as a good preventative measure, if possible divided into two doses.
This is all based on my experience. I am not a medical doctor so nothing here is meant to constitute medical advice.