A combination of chemotherapy and radiation is often used to treat rectal cancer patients before surgery in an effort to shrink the tumor and make it easier to remove as well as to help increase the chances of sphincter-sparing surgery. Many previous clinical studies have suggested that rectal cancer patients may survive longer if the surgery results in a pathological complete response - that is, the absence of any tumor cells in the surgical specimen. However, there is still controversy over this. This study attempts to start to answer this question by treating rectal cancer patients with a combination of chemotherapy drugs (oxaliplatin and capecitabine), a cyclooxygenase-2 (COX-2) enzyme inhibitor and radiation before surgery. The rates of pathologic complete response, sphincter-sparing surgery, and disease-free survival are some of the therapeutic endpoints that will be studied.