Hi I've tested on:
Tower, Windows Vista Ultimate SP1, AutoCAD 2010, Bricscad Pro v10
Laptop, Windows XP SP3, AutoCAD 2007 and Bricscad Pro v10
All issues were replicated with one difference:-
On the Vista machine - SlideControl MouseDown Event doesnot show the textbox either, however when the mouse is hoverred over it, it appears and indeed the text has been added to it (unlike for the XP machine where the textbox remains blank).