Hi again. Thanks for the additional info.
You note that harassment is illegal, but that's only partially true. Whether or not it's illegal depends on the motivation of the harassment. For example, if you were mistreated because the co-worker doesn't like your race, ethnicity, gender or disability, then you may indeed have recourse. What you describe would be illegal discrimination. The first step in dealing with illegal discrimination is to file a charge with the Equal Employment Opportunity Commission (EEOC). The EEOC is responsible for investigating illegal discrimination in the workplace. Once the EEOC investigates, it could agree that illegal discrimination took place, and attempt to work out a settlement with the employer. If the EEOC cannot work out a settlement, or if the EEOC does not find that there was illegal discrimination, then a person can request a right to sue letter, which gives her the right to sue her employer for violating the law. You can find out how to file a charge here: http://www.eeoc.gov/employees/charge.cfm
On the other hand, if the co-worker mistreated you simply because she doesn't like your personality, then that would not be illegal. The law does not require politeness or that co-workers get along with each other. It's up to the employer to enforce any policies (or no policies) regarding those types of issues.
So, in sum, the reason for the mistreatment is extremely important, and determines whether you have recourse.
Does that answer your question? Please let me know if you need clarification, as I am happy to continue helping you until you are satisfied.