I've heard that you are most contagious in the few days before you come down with any symptoms, but i'm not sure if you are non-contagious during the actual period of suffering or not. Anyone know for sure?
Most of these bugs are airborne viruses, so all the time you are spreading them into the air, by sneezing or blowing your snotty nose, you're contagious. Often, though, with these things, you have a few sneezes a day or two before you start to feel really lousy, yet you're still spreading the bugs. Plus, as the mouth is part of the respiratory tract, if you share a drink or let someone have a bite of your food, you can share germs that way too.
A person is contagious from the day before the illness breaks out until one to three days after they feel better. The infection is spread by airborne droplets when the sufferer coughs or sneezes. It can also be spread by hand if someone has the virus on their hands and then puts them close to their eyes or nose. This is possibly the most common way of catching a cold.
Loads of stuff about colds on this link
http://www.netdoctor.co.uk/diseases/facts/colds.ht m