Having a cold sucks. Your tired, snotty, your throat hurts and your head feels like its filled with sand. As soon as I get ill all I can think about is getting better as fast as possible. I have tried dozens of remedies for curing my colds including overloading my Vitamin C, eating cloves of raw garlic (my girlfriend loved that), hot lemon tea and ginger mixes and even gargling with Tabasco sauce. The only thing I know that works is rest and time.
My major gripe about getting sick is that I cannot train during this time. I do this because I have learned from several past experiences that any type of difficult or strenuous activity made the cold far worse. Normally I can beat a cold in about 3 days, but if I was stupid enough to hit the gym it would get extended another several days. I have read on a few websites that an intense workout will help flush out a cold. The only thing intense your going to get is intensely sick. Do yourself a favor when you are suffering from a cold or flu and just take a break. Your body will thank you for it and you will get better far quicker.
If you absolutely have to exercise though, I found that very low intensity cardio could be done without making the cold worse. This could include walking or a low speed session on a cross-trainer for under 3o minutes.