You can use the Line method in the report's Page event procedure ...
Private Sub Report_Page()
Dim sngTop As Single, sngLeft As Single
Dim sngWidth As Single, sngHeight As Single
Me.ScaleMode = 3
sngTop = Me.ScaleTop + 5
sngLeft = Me.ScaleLeft + 5
sngWidth = Me.ScaleWidth - 10
sngHeight = Me.ScaleHeight - 10
Me.Line (sngTop, sngLeft)-(sngWidth, sngHeight), vbBlack, B
End Sub
See Line Method in the help files for more information.