You should sit down with an attorney and discuss your situation. Most attorneys offer a free consultation and would likely be able to give you at least a rough idea of what your rights and obligations are here.
It appears we are unanimous. You need to consult with an attorney with experience in dealing with employment and noncompete agreements. Based on your statement of the facts, it is difficult to assess your chances of success. For anyone else who might be reading this, I will tell you that no employee should ever sign a non-compete agreement as a condition of employment without first consulting an attorney. Generally it is OK to sign an agreement to not transfer or mis-use trade secrets and other proprietary information--but giving up your right to obtain other employment in your field is almost always a bad idea.