Keep your cold at home:
The Straight Dope Mailbag: How long are colds contagious?: "The mode of transmission is presumed to be by direct contact or by inhalation of airborne droplets, or indirectly (and more importantly) by hands and coming in contact with anything contaminated by snot or spit from an infected person. Shake hands or use the telephone right after that sneezing secretary, then touch your eyes, and you've got it, babe. Depending on the specific germ, the incubation period is between 12 hours and 5 days, usually 48 hours.
To answer your question, the common cold is 'contagious' between 24 hours before onset of symptoms until 5 days after onset. Like everything else this varies depending on the organism, but that's a pretty safe estimate. This period of communicability was determined by taking nasal washings of experimentally infected volunteers. There is also the possibility of healthy carriers, though this appears to be rare with rhinoviruses."