Surely you mean I-bar, not the mouse cursor?
It's position and length are controlled by the selstart and sellength properties. You could count the number of line feed characters up to that point to determine the current line, and then set the position the hard way but why not just:
Private Sub Text1_KeyDown(KeyCode as Integer, Shift as Integer)
If KeyCode = vbKeyLeft then KeyCode = vbKeyHome
End Sub