The human body has a natural stress response to surgery, including the formation of blood clots. This response to surgery has been shown to increase metastases (the spread of cancer cells to other organs in the body). These metastases cannot be seen at the time of surgery but when they grow into new tumors, the cancer has recurred (come back). A blood thinner called "low molecular weight heparin" (LMWH) can suppress the development of metastases after surgery in animal experiments. The investigators want to see if giving patients with colorectal cancer the blood thinner, LMWH, around the time of surgery can decrease the chance of their cancer spreading to other organs (metastases) and coming back (recurrence).