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At least in Excel 2007 a simple formula is

=TODAY()-WEEKDAY(TODAY(),3)

The 3 tells WEEKDAY to number the week as Monday=0,Tuesday=1...Sunday=6

If you need a macro

Sub ReturnCurrentMonday()
'Return Monday of current week.
Dim dteMonday As Date
dteMonday = Date - Weekday(Date, vbMonday) + 1
Selection.Value = dteMonday
End Sub

will put Monday's date in the currently selected cell