While the common cold is usually a nuisance, it can lead to other problems. Most colds should pass in 4-5 days provided you rest, however if the linger this may be indicative of another problem. When you have a cold be sure to rest (take time out of work if possible), drink plenty of fluids and use some vitamin C. Also do your best to decongest. Medicines like guaifenesin help liquefy mucous to clear it. Be careful of pseudoephedrine as it can cause hypertension, insomnia and anxiety!
If your cold lingers past 1 week, you may have developed a secondary problem. Sinusitis, bronchitis and ear infections can result, as well as pneumonia if you are unlucky. Seek medical attention of you feel these conditions have developed!