Date and Time Setting in Windows XP

How can i change date setting of Windows Xp. Actually when i used excel and put any date by the format of Day-month-year the excel automatically changed it into Month-Day-Year. I checked this option in the excel but the format of date in excel was correct as per my need. I also observed that time in task bar was showing without AM and PM. How can i change setting of time and date as per my requirement.
