Can I win a case against a former Employer who is threatning to sue me if I dont quit my current job because its a competitor?

Non-compete provisions in employment agreements are sometimes enforceable, but not always.  It depends on a lot of factors, including how broad the restriction is (i.e. does it prevent the former employer from any competition or just from soliciting his former employer's customers, or just from doing business with customers he serviced during his last year working for his former employer, etc.), how long the restriction lasts, how geographically broad the restriction is, whether the employee has specialized skils or is just doing a job which many people can do, whether the employee is using trade secrets or confidential information from his old employer in his new job, whether the employee was paid anything extra for not competing, etc.  Also, in some cases (generally where a non-compete clause is deemed just a little overbroad but not really extreme), a Court will "blue pencil" an otherwise unenforceable non-compete provision by narrowing it so it is acceptable, e.g. changing a clause which prohibits competition within ten miles for three years to one which prohibits competition within 2 miles for 1 year.