From MSDN:
By using the Microsoft Windows® application programming interface (API) SetScrollRange function, you can add code to your Visual Basic application that will allow you to create a work-around solution. This enables you to change the Text Box's appearance from single to multiline at run time.
The SetScrollRange function lets you set the minimum and maximum indicator positions of a scroll bar. To use this function, add the following Declare statement to the General Declarations section of your form (note that the Declare statement must be typed as a single line of code):
Private Declare Sub SetScrollRange Lib "User" (ByVal hWnd As Integer, ByVal nBar
As Integer, ByVal nMinPos As Integer, ByVal nMaxPos As Integer, ByVal
bRedraw As Integer)