How can i stop my 2 1/2 yr old boy to stop hittin my 5 yr old daughter (will be 5 in DEC. 2007)?

Don't encourage it by telling her to hit him back! Take away what's near and dear to him. If that doesn't do it, take away every single toy that he owns and tell them he can earn them back one toy for every day that he makes it without hitting. That outta do it.