I've always heard you can do some low impact exercise (walking, easy bicycling, etc.) when you have a cold as long as you're not running a fever. If you're running a fever you should NOT exercise, as your body is already working overtime trying to fight your infections. I sometimes just walk slowly on the treadmill just to do something when I'm sick. It isn't the cardio we all need and love (lol), but its still getting my heart rate up higher than sitting on the couch, and it also makes it easier to get back into exercise once you are feeling better, since you haven't just sat around the whole time you were ill.
Hope you feel better soon!!!