Thanks Chris, I have figured it out now:

Declare @From Datetime

Declare @To Datetime

Set @From = '07 Jan 2013'

Set @To = '18 Mar 2013'

SELECT Datediff(Week, dateadd(dd, CASE WHEN datepart(weekday, @From) = 1 THEN 0 ELSE 8 - datepart(weekday, @From) END,@From),

dateadd(dd, CASE WHEN datepart(weekday, @To) = 1 THEN 0 ELSE 1 - datepart(weekday, @To) END,@To))+1