As you might have noticed DateTime does not have a WeekNumber property. This however is available inside the CultureInfo calendar :)
So we first create a culture variable and fill it with the current culture.
Then you can first configure some more rules inside the calendar like what day of the week it should concider as the first day.
Here a code example for you :)
Good luck
DateTime dateTime = DateTime.Parse("08-03-2010");
System.Globalization.CultureInfo culture = System.Globalization.CultureInfo.CurrentCulture;
int intWeek = culture.Calendar.GetWeekOfYear(dateTime, System.Globalization.CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);