How to delete oldest week(period) in MS SQL database?

Question

MS SQL 2008R2 Express, I have database - one table with rows. And when db size became about 10gb I need to clean one oldest week in this table (I dont know the date). Could you help me with script? Thanks.

UPD1. There is column DateAndTime, so i make query like this in VBA:

strSQL = "SELECT DateAndTime
                ,TagName
                ,Val
                ,SetPoint
                ,Limit_H
                ,Limit_L
                ,Result 
          FROM dbo.Statistic 
          WHERE DateAndTime  BETWEEN CAST('" & TimeBegin & "' AS datetime) AND CAST( '" & TimeEnd & " ' AS datetime) 
          Order By DateAndTime desc , (CASE WHEN ISNUMERIC(TagName)=1 THEN CAST(CAST(TagName AS float) AS INT)END ) desc"

Answer 1

What about this -

DELETE YOUR_TABLE
WHERE DATE_COL BETWEEN (SELECT MIN(DATE) FROM YOUR_TABLE) AND (SELECT MIN(DATE) FROM YOUR_TABLE) + 7

Answer 2

As Ankit suggested, use the min() of the date and add a week

delete 
from dbo.statistic
where dateandtime between min(dateandtime) 
                    and dateadd(week, 1, min(dateandtime))